THE    GREAT    TELESCOPE    OF    THE    UNITED    STATES    NAVAL    OBSERVA- 
TORY,  WASHINGTON. 

CONSTRUCTED    BY    ALVAN    CLARK    ANU    SONS,   ,.  373. 


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POPULAR  ASTRONOMY. 


BY 


SIMON    NEWCOMB,   LL.D., 

PUOFESSOK,   V.  S.  NAVAL   OBSEUVATOKY. 


WITH  ONE  HUNDRED  AND   TWELVE  ENGRAVINGS, 
AND  FIVE  MAFS  OF  THE  STARS. 


<^5Ii£i5 


oii 


LIBRARY 


NEW    YORK: 

HARPER    &    BROTHERS,    PUBLISHERS, 

FRANKLIN    SQUARE. 

1878. 


Entered  according  to  Act  of  Congress,  in  the  year  1877,  by 

11  A  U  P  E  R      &      B  U  O  T  H  E  U  S, 

In  the  Office  of  the  Librarian  of  Congress,  at  Washington. 


PREFACE. 


To  prevent  a  possible  misapprehension  in  scientific  (piar- 
ters,  the  author  desires  it  understood  that  the  present  work 
is  not  designed  either  to  instruct  tlie  professional  investi- 
gator or  to  train  the  special  student  of  astronomy.  Its  main 
object  is  to  present  the  general  reading  public  with  a  con- 
densed view  of  the  history,  methods,  and  results  of  astro- 
nomical research,  especially  in  those  tields  which  are  of  most 
popular  and  philosophic  interest  at  the  present  day,  couched 
in  such  language  as  to  be  intelligible  without  mathematical 
study.  He  hopes  that  the  earlier  chapters  will,  for  the  most 
part,  be  readily  understood  by  any  one  having  clear  geomet- 
rical ideas,  and  that  the  later  ones  will  be  intelligible  to  all. 
To  diminish  the  difficulty  which  the  reader  may  encounter 
from  the  unavoidable  occasional  use  of  technical  terms,  a 
Glossary  has  been  added,  including,  it  is  believed,  all  that 
are  used  in  the  present  work,  as  well  as  a  number  of  others 
which  may  be  met  with  elsewhere. 

Respecting  the  general  scope  of  the  work,  it  may  be  said 
that  the  historic  and  philosophic  sides  of  the  subject  have 
becxi  treated  with  greater  fulness  than  is  usual  in  works  of 
this  charactei",  while  the  purely  technical  side  has  been  pro- 
portio!iately  condensed.  Of  the  four  parts  into  which  it  is 
divided,  the  first  two  treat  of  the  methods  by  which  the  mo- 


VI  PREFACE. 

tions  and  the  mutual  relations  of  the  heavenly  bodies  have 
l)een  investigated,  and  of  the  results  of  sueh  investigation, 
while  in  the  last  two  the  individual  peculiarities  of  those 
bodies  are  considered  in  greater  detail.  The  subject  of  the 
general  structure  and  ])robable  development  of  the  universe, 
which,  in  strictness,  might  be  considered  as  belonging  to  the 
iirst  part,  is,  of  necessity,  treated  last  of  all,  because  it  re- 
(piires  all  the  light  that  can  be  thrown  upon  it  from  every 
available  source.  Matter  admitting  of  presentation  in  tabular 
form  has,  for  the  most  part,  been  collected  in  the  Appendix, 
where  will  be  found  a  number  of  brief  articles  for  the  use 
of  both  the  general  reader  and  the  amateur  astronomer. 
4  The  author  has  to  acknowledge  the  honor  done  him  by 
several  eminent  astronomers  in  making  his  work  more  coin- 
plete  and  interesting  by  their  contributions.  Owing  to  the 
great  interest  which  now  attaches  to  the  question  of  the  con- 
stitution of  the  sun,  and  the  rapidity  with  wdiich  our  knowl- 
edge in  this  direction  is  advancing,  it  was  deemed  desirable 
to  present  the  latest  views  of  the  most  distinguished  investi- 
gators of  this  subject  from  their  own  pens.  Four  of  these 
gentlemen — Eev.  Father  Secchi,  of  Home ;  M.  Faye,  of  Paris ; 
Professor  Young,  of  Dartmouth  College ;  and  Professor  Lang- 
ley,  of  Alleglieny  Observatory — have,  at  the  author's  request, 
presented  brief  expositions  of  their  theories,  which  will  be 
found  in  their  own  language  in  the  cha2:)ter  on  the  sun. 

An  Addendum  gives  the  basis  of  the  remarkable  modifi- 
cation of  the  theory  of  the  solar  spectrum  proposed  by  Dr. 
Henry  Draper,  which  appeared  while  the  sheets  were  passing 
through  the  press. 


CONTENTS. 


PART   I. 

THE  SYSTEM  OF  THE    WORLD  HISTORICALLY  DEVELOPED. 

I'ACiK 

Intuoduction 1 

CHAPTER  I. 

The  Anciknt  Astuonomy,  on  the  Ai'i-akent  Motions  of  the  Heav- 
enly Bodies 7 

§1.  Tlie  Celestial  Sphere 7 

§2.  Tlie  Diurnal  Motion 5) 

§3.  Motion  of  the  Sun  among  the  Stars 13 

§  4.  Precession  of  tlie  Equinoxes. — The  Solar  Year ID 

§0.  Tlie  Moon's  Motion..... 121 

§  (■).   Eclipses  of  the  Sun  and  Moon 24 

§  7.  Tiie  Ptolemaic  System 32 

§8.  The  Calendar 44 

CHaPTF.R   II. 

The  Copernican   System,  or  the  True   Motions  or  the   Heavenly 

Bodies ill 

§  1.  Copernicns 51 

§  2.  Obliquity  of  the  Ecliptic  ;  Seasons,  etc. ;  on  the  Copernican  Sys- 
tem   (J  I 

§3.  Tycho  Brahe GfP 

§4.  Kepler. — His  Laws  of  Planetary  Motion G8 

§.5.  From  Kepler  to  Newton , 71 


viii  CONTEMS. 

CIIAPTKK  HI.  PAOK 

Univehsal  Guavitation 74 

§1.  Newton. — Uiscovery  of  Gravitation 74 

§2.  Gravitation  of  ISmail  Mas.ses. — Density  of  tlie  Earth 81 

§3.  Figure  of  the  Eartli , 8(> 

§4.   Precession  of  the  Eqnino.xes 88 

§  5.  The  Tides <)0 

§  (>.   Ineciualities  in  the  Motions  of  the  Planets  produced  by  tiieir 

Mutual  Attraction y;5 

§7.   Relation  of  the  Planets  to  the  Stars 101 


TART  II. 

PEACTICAL  ASTllONOMY. 
Introductory  Remarks lO;^ 

CHAPTER  I. 

The  Telescope lOO 

§1.  The  First  Telescopes lOg 

§  2.  The  Achromatic  Telescope 114 

§3.  The  Mounting  of  the  Telescope 118 

§4.  The  Reflecting  Telescope 121 

§  5.  The  Principal  Great  Reflecting  Telescopes  of  Modern  Times...  125 

§  G.  Great  Refracting  Telescopes I35 

§  7.  The  Magnifying  Powers  of  the  Two  Classes  of  Telescopes 139 

CHAPTER  II. 

Application  of  the  Telescope  to  Celestial  Measurement!: 14G 

§  1.  Circles  of  the  Celestial  Sphere,  and  their  Relations  to  Positions 

on  the  Earth 14C, 

§2.  The  Meridian  Circle,  and  its  Use 152 

§3.  Determination  of  Terrestrial  Longitudes 157 

§4.  Mean,  or  Clock,  Time 102 


CONTENTS.  ix 

CIIAl'TER   III.  PA^K 

Measciunc  Distances  in  tiik  IIkavkns 1G5 

§1.  Parallax  in  General 1(J5 

§2.  Mensurcs  of  the  Distance  of  the  Sun 171 

§3.  Solar  Parallax  from  Transits  of  Venus 175 

§  4.  Other  Methods  of  Determining  the  Sun's  Distance,  and  their 

lles'ilts 15)4 

§5.  Stellar  Parallax 201 


CHAl'TKU   IV. 
Tiiii  Motion  of  LiciUT 210 

CHAPTER  V. 
TiiE  Spectroscope 222 


PAET  III. 

TEE  SOLAR  SYSTEM. 


CHAPTER   I. 
General  Strocture  of  the  Solar  System 231 

CHAPTER  II. 

The  Sun 237 

§1.  The  Photosphere 237 

§2.  The  Solar  Spots  and  Rotation 24? 

§3.  Periodicity  of  the  Spots 248 

§4.  Law  of  Rotation  of  the  Sun 241) 

§  5.  The  Sun's  SuiToundings. — Phenomena  of  Total  Eclipses 2.")! 

§  G.  Physical  Constitution  of  the  Sun 2r)8 

§  7.  Views  of  Distinguished  Students  of  the  Sun  on  the  Subject  of 

its  Physical  Constitution 205 


X  CONTENTS. 

CIIAPTEIl     III.  PAGE 

The  Inner  Guocp  of  Plankts l'83 

§1.  The  rianet  Merciiiy 283 

§2.  The  Supposed  liitrii-Meiciirial  rUiiiets 28G 

§3.  The  rhmet  Venus 28!) 

§4.  The  Earth 298 

§5.  The  Moon 30G 

§0.  The  rUinet  Mars o2() 

§7.  The  Small  Planets 323 

CHAPTER   IV. 

The  Outkr  Group  of  Pl.^nlts 331 

§1.  The  Planet  Jupiter 331 

§2.  The  Satellites  of  Jupiter 336 

§3.  Saturn  and  its  System,  Physical  Aspect,  Belts,  Kutation 338 

§4.  The  Pings  of  Saturn 341 

§  T).  Constitution  of  the  Ring 34i> 

§  G.  The  Satellites  of  Saturn 351 

§7.  Uranus  and  its  Satellites 353 

§8.  Neptune  ana  its  Satellite 358 

CHAPTER  V. 

Comets  and  Mftkors 3G5 

§  1.  Aspects  and  Forms  of  Comets 3G5 

§2.  Motions,  Origin,  and  Xumber  of  Comets 3G9 

§3.  Remarkable  Comets 374 

§4.  Encke's  Comet,  and  the  Resisting  Medium 381 

§5.  Meteors  and  Shooting- stars 384 

§  G.  Relations  of  Comets  and  Meteoroids 391 

§7.  The  Physical  Constitution  of  Comets 398 

§8.  The  Zodiacal  Light 405 


PART   IV. 

THE  STELLAR    UNIVERSE. 
Introductory  Remarks 407 


CO^'TEXTS.  XI 

CHAPTER   I.  PAOE 

The  Stars  as  tiiev  are  Seen 410 

§  I.  Number  and  Orders  ot  Stars  and  Nelmlai 410 

§2.  DesLiiption  of  the  Principal  Ccustellations 417 

§  3.  New  and  Variable  Stars 42(5 

§4.  Double  Stars 43G 

§5.  Clusters  of  Stars 441 

§  (!    Nebula; 444 

§  7.  Proper  Motions  of  the  Stars 452 

CHAPTER   II. 

The  Structure  of  the  Universe 460 

§1.  Views  of  Astronomers  before  Herschel 4G1 

§  2.  Researches  of  Herschel  and  his  Successors 4G5 

§  3.   Probable  Arrangement  of  the  Visible  Universe 478 

§4.  Do  the  Stars  really  form  a  System? 483 


CHAPTER  III. 

The  Cosmogony 491 

§1.  The  Modern  Nebular  Hypothesis 403 

§  2.  Progressive  Changes  in  our  System iW 

§3.  The  Sources  of  the  Sun's  Heat 505 

§4.  Secular  Cooling  of  the  Earth 511 

§5.  General  Conclusions  respecting  the  Nebular  Hypothesis 514 

§  G.  The  Plurality  of  Worlds 5IG 

Addendum  to  Part  III.,  Chapter  II 520 


APPENDIX. 

I.  List  of  the  Pr'ncipal  Great  Telescopes  of  the  World 521 

II.  List  of  the  moke  Remakkahle  Douhle  Stars 523 

III.  List  of  the  more  Interesting  and  Remarkahle  Nehul.?:  and 

Star  Clusters 525 

IV.  Periodic  Comets  seen  at  more  than  One  Return 527 


XU  CONTEXTS. 

PAOE 

V".  Elements  of  the  Orhits  of  the  Eight  Majou  Planets  for  1850.  fi28 

Elements  of  the  Satellites  oi'  Jopitek 52!) 

Elements  of  the  S.^tellites  of  Satukn r)2'J 

Elements  op  the  Satellite  of  Neptune 52!) 

Elements  of  ihe  Satellites  of  Uranus 52!» 

VI.  Elements  of  the  Small  Planets 530 

VII.  Detekmin.^t'ons  of  Stellar  Parallax 535 

VIII.  Synopsis  of  Papers  on  the  Solau  Parallax,  185-t-'77 538 

IX.  List  of  AsiuoNt)MiCAL  Works,  imost  of  which  have  ijeen  con- 
sulted AS  Authorities  in  the  Preparation  of  the  Present 

Work 542 

X.  Glossary'  of  Technical  Terms  of  Frequent  Occurrence   in 

As  1  konomic al  Works , 549 

Index 55!) 

Addendum  II. — The  Satellites  of  Mars 5G5 

Explanation  of  the  Star  Maps 568 


LIST  OF  ILLUSTRATIONS. 


no.  PAOE 

The  Gkkat  Telescope  of  the  TIn'ted  States  Naval  Observato- 
KY,  Washington Frontispiece 

1.  Section  of  the  Imaginary  Celestial  Sphere 8 

2.  Map  illustrating  the  Diurnal  Motion  round  the  Pole 10 

;}.  The  Celestial  Sphere  and  Diurnal  Motion 12 

4.  Motion  of  the  Sun  past  the  Star  Regulus 15 

Ti.  Showing  the  Sun  to  be  farther  than  the  Moon 22 

(J.  Annular  Eclipse  of  the  Sun...  2G 

7.  Partial  Eclipse  of  the  Sun 2G 

8.  Eclipse  of  the  Sun,  the  Shadow  of  the  Moon   falling  on  the 

Earth 2G 

9.  Eclipse  of  the  Moon,  in  the  Shadow  of  the  Earth 27 

10.  Showing  the  Apparent  Orbit  of  a  Planet 38 

11.  Apparent  Orbits  of  Jupiter  and  Saturn 39 

12.  Arrangement  of  the  Seven  Planets  in  the  Ptolemaic  System...  41 

13.  The  Eccentric 42 

14.  Showing    the    Astrological    Division    of    the    Seven    Planets 

AMONG    THE    DaYS    OF    THE    WeEK 4G 

15.  Apparent  Annual  Motion  of  the  Sun  explained 55 

IG.  Showing  how  the  Apparent  Epicy'CLic  Motion  of  the  Planets 

is  accounted  for 5G 

17.  Relation  of  the  Terrestrial  and  Celestial  Poles  and  Equators.  G2 

18.  Causes  of  Changes  of  Seasons  on  the  Copernican  System G3 

19.  Enlarged  View  of  the  Earth,  showing  Winter  in  the  North- 

ern   HiCMISPHERE,  AND    SuMMER    IN    THE    SOUTHERN G5 

20.  Illustrating  Kepler's  First  Two  Laws  of  Planetary  Motion...  G9 

21.  Illustrating  the  1'all  of  the  Moon  towards  the  Earth 78 

22.  Bailv's  Apparatus  for  determining  the  Density  of  the  Earth.  83 

23.  View  of  Daily's  Apparatus 84 

24.  Diagram  illustrating  the  Attraction  of  Mountains  85 

25.  Precession  of  the  Equinoxes 88 


xiv  LIST  OF  ILLUSTIiATWXS. 

26.  Attuaction  of  thk  Moon  tkndino  to  raonucE  Tides !)1 

27.  Akmillary  Spheke  as  descriiieu  hy  Ttolemv 105 

28.  The  Galilean  Telescoi-e 108 

29.  FOKJIATION    OF    AM    I.MAGE    liV    A    LeNS 109 

;10.    GUEAT    TELESCOfi:    OF    THE    SEVENTEENTH    CeNTCUY 112 

31.  Refuaction  thkougi'  a  Compound  Puism 114 

;{2.  Section  of  an   Achromatic  Ohjective 115 

33.  Section  of  Eye-piece  of  a  Telescope 118 

34.  RioDE  OF  Mounting  a  Telescope 119 

35.  Speculum  Bringing  1  .is  to  a  Single  Focus  in  Reflection 122 

36.  IIerschelian  Telescope 123 

37.  HoRi^.oNTAL  Section  of  a  Newtonian  Tei.escope 123 

38.  Section  of  the  Gregorian  Telescope 124 

39.  Herschel's  Great  Telescope 127 

40.  Lord  Rosse's  CJreat  Telescope 130 

41.  Mr.  Lassell's  Great  Four-foot  Reflector 132 

42.  The  New  Paris  Reflector 134 

43.  The  Great  Melhournf,  Reflector 136 

44.  Circles  of  the  Celestial  Sphere 147 

45.  The  Washington  Transit  Circle 153 

46.  Spider  Lines  in  Field  of  View  of  a  Meridian  Circle 154 

47.  Diagram  illustrating  Parallax 165 

48.  Diagram  illustrating  Parallax 166 

49.  Variation  of  Parallax  with  the  Altitude 167 

50.  Apparent  Paths  of  Venus  across  the  Sun 176 

51.  Venus  approaching  Internal  Contact  on  the  Face  of  the  Sun.  178 

52.  Internal  Contact  t)F  Limh  of  Venus  with  that  of  the  Sun 178 

53.  The  Black  Drop,  or  Ligament 179 

54.  Method  of  Photo(;kaphing  the  Transit  of  Venus 186 

5.5.  Artificial  Transit  of  Venus 188 

56.  Map  of  the  Earth,  showing  the  Areas  of  Visibility  of  the 

Transit  of  1874 191 

57.  Map  of  the  World,  showing  the  Regions  in  avhich  the  Tran- 

sit OF  Venus  will  he  visihle  on  Decemher  6th,  1882 195 

58.  Effect  of  Stellar  Parallax 202 

59.  Aberration  of  Light 212 

60.  Revolving  Wheel  for  measuring  the  Velocity  of  Light 216 

61.  Illusthating  Foucault's  Method   of  measuring  the  Velocity 

OF  Light 218 

62.  Course  of  Rays  through  a  Spkctkoscope 224 


LIST  OF  ILLUSTRATIONS.  XV 

FIG.  VAOR 

iVA.  Rklativk  Sizk  of  Sun  and  Pi.ankts 232 

(!4.  Okhits  ok  the  Planets  from  tiik  Eauth  outwaud 230 

Go.  Man  iioLoiNd  Tei.kscoi'k,  to  show  Sun  on  Scki;i;n 2415 

()G.  SoLAU  Spot,  afteu  Siccchi 244 

G7.  Changes  in  the  Aspect  of  a  Solaii  Spot  as  it  cuosses  the  Sun's 

Disk 24G 

G8.  Total  Eclipse  of  the  Sun,  as  seen  at  Des  Moines,  Iowa,  Au- 
gust 7th,  ISGO 253 

GD.  Specimens  of  Solau  Puotuhkkances,  as  dua\vn  hy  Secciii 25G 

70.  The   Sun,  with   its   Chromospheke   and   Red   Flames,  on  July 

23i),  1871  2G1 

71.  Illustrating  Secchi's  Theory  of  Solar  Spots 2G9 

72.  Solar  Spot,  after  Langley 281 

73.  Orhits  of   the  Four  Inner  Planets,  illustrating  the   Eccen- 

tricity of  those  of  Mercury  and  Mars 283 

74.  Phases  of  Venus 201 

75.  Showing  the  Thickness  of  the  Earth's  Crust 299 

7G.  DlSTRIIlUTION  OF  AURORAS 302 

77.  View  of  Aurora 303 

78.  Spectrum  of  Two  of  the  Great  Auroras  of  1871  305 

79.  Kelative  Size  of  Earth  and  Moon 30G 

80.  View  of  Moon  near  the  Third  Quarter 313 

81.  I/UNAR  Crater  "Copernicus" 315 

82.  The  Planet  Mars  on  June  23d,  1875 322 

83.  Map  of  Mars 322 

84.  Northern  Hemisphere  of  Mars 323 

85.  Southern  Hemisphere  of  Mars 323 

8G.  Jupiter,  as  seen  with  the  Great  Washington  Telescope,  March 

21sT,  187G 331 

87.  View  of  Jupiter,  as   seen  in  Lord   KorsES  Great  Telescope, 

Fekruary  27th,  18G1 333 

88.  ViEAV  OF  Saturn  and  his  Rings 339 

89.  Specimens  of  Drawings  of  Saturn  ijy  Various  Observers 343 

90.  Views  of  Encke's  Comet  in  1871 3G7 

91.  Head  of  Donati's  Great  Comet  of  1858 3G8 

92.  Parabolic  and  Elliptic  Orbit  of  a  Comet 370 

93.  Orbit  of  Halley's  Comet 377 

94.  Great  Comet  of  1858 380 

95.  Meteor  Paths,  illustrating  the  Radiant  Point 390 

9G.  Orbit  of  November  Meteors  and  the  Comet  of  18G1  394 


XVI 


LIST  OF  ILLUSTRATIONS. 


V\a.  PAOK 

97.  Orkit  of  Tiir  TiniU)  Comkt  of  18G2 395 

98.  Measurk  of  Position  ANtu-E  of  Doijiu.e  Star 438 

99.  Distance  of  Component*;  of  Dourle  Star 438 

100.  Diagram  to  illustiute  1'osttion  Akcm-e 438 

101.  Telescopic  Vieav  of  the  Pleiades 442 

102.  Cluster  of  i7  Toucani 444 

103.  Cluster  w  Centauri 444 

104.  The  Great  Nerula  of  Orion 44(5 

10r>.  The  Annular  Nerula  in  Lyra 448 

100.   The  Omega  Nerula 450 

107.  Nerula  Herschel  3722 451 

108.  The  Looped  Nerula;    Herschel  2!)4I 451 

109.  Herschel's  View  of  the  Form  of  the  Universe 4G9 

110.  Illustrating  Herschel's  Orders  of  Distance  of  the  Stars 471 

111.  Probahlk   Arrangement   of   the    Stars    and   Nebula:      isirle 

WITH  THE  Telescope 481 

112.  Diagram  illustrating  Elliptic  Elements  of  a  Planet 551 


STAR  MAPS, 


Map      L — The   Northern   Constellations   within  50^" 
OF  THE  Pole 

"      IL — Southern    Constellations    Visible    in    Au- 
tumn AND  Winter 

"     in. — Southern  Constellations  Visible  in  Win- 
ter AND  Spring 

"     IV. — Southern  Consteli,ations  Visible  in  Spring  j 
AND  Summer I 

"       V. — Southern   Constellations   Visible  in  Sum-  | 
mer  and  Autumn J 


)■  At  End  of  Book. 


POPULAR  ASTROI^OMY. 


PART  L  —  TIIE  SYSTEM  OF  THE   WORLD 
HISTORICALLY  DEVELOPED. 


INTKODUCTIOK 


Astronomy  is  the  most  ancient  of  the  physical  sciences,  be- 
ing distinguished  among  them  by  its  slow  and  progressive 
development  from  the  earliest  ages  until  the  presetit  time. 
In  no  other  science  lias  each  generation  which  advanced  it 
been  so  much  indebted  to  its  predecessors  for  both  the  facts 
and  the  ideas  necessary  to  make  the  advance.  The  conception 
of  a  globular  and  moving  earth  pursuing  her  course  through 
the  celestial  spaces  among  her  sister  planets,  which  we  see  as 
stars,  is  one  to  the  entire  evolution  of  which  no  one  mind  and 
no  one  age  can  lay  claim.  It  was  the  result  of  a  gradual 
process  of  education,  of  which  the  subject  was  not  an  indi- 
vidual, but  the  human  race.  The  great  astronomei-s  of  all 
ages  have  '  lilt  upon  foundations  laid  by  their  predecessors ; 
and  when  we  attempt  to  search  out  the  first  founder,  we  find 
ourselves  lost  in  the  mists  of  antiquity.  The  theory  of  uni- 
versal gravitation  was  founded  by  Newton  upon  the  laws  of 
Kepler,  the  observations  and  measurements  of  his  French  con- 
temporaries, and  the  geometry  of  ApoUonius.  Kepler  used 
as  his  material  the  observations  of  Tycho  Brahe,  and  built 
upon  the  theory  of  Copernicus.  When  we  seek  the  origin  of 
the  instruments  used  by  Tycho,  we  soon  find  oui-selves  among 

2 


2  SYSTEM  OF  THE  WORLD  HISTORICALLY  DEVELOPED. 

the  iiicdian-d  Arabs.  The  discovery  of  the  true  system  of 
tlie  vv^orld  by  Copernicus  was  only  possible  by  a  careful  study 
of  the  laws  of  apparent  motion  of  the  planets  as  expressed  in 
the  epicycles  of  rtolcniy  and  Ilipparchus.  Indeed,  the  more 
carefully  one  stur^ies  the  great  work  of  Copernicus,  the  more 
surprised  he  will  ,  to  find  how  coinpletely  Ptolemy  furnished 
him  both  ideas  and  material.  If  we  seek  the  teachers  and 
predecessors  of  Ilipparchus,  we  find  only  the  shadowy  forms 
of  Egyptian  and  Babylonian  priests,  whose  names  and  writings 
are  all  entii-ely  lost.  In  the  earliest  historic  ages,  men  knew 
that  the  earth  was  round ;  that  the  sun  appeared  to  make  an 
annual  revolution  among  the  stars;  and  that  eclipses  were 
caused  by  the  moon  entering  the  shadow  of  the  earth,  or  the 
earth  that  of  the  moon. 

Indeed,  each  of  the  great  civilizations  of  the  ancient  world 
seems  to  have  had  its  own  system  of  astronomy  strongly 
marked  by  the  ^^  iculiar  character  of  the  people  among  whom 
it  was  found.  Several  events  recorded  in  the  annals  of  China 
show  that  the  movements  of  the  sun  and  the  laws  of  eclipses 
were  studied  in  that  country  at  a  very  early  age.  Some  of 
these  e\ents  mnst  be  entirely  mythical ;  as,  for  instance,  the 
despatch  of  astronomers  to  the  four  points  of  the  compass  for 
the  purpose  of  determining  the  equinoxes  and  solstices.  But 
there  is  another  event  which,  even  if  we  place  it  in  the  same 
category,  mnst  be  regarded  as  indicating  a  considerable  amount 
of  astronomical  knowledge  among  the  ancient  Chinese.  We 
refer  to  the  tragic  fate  of  Hi  and  IIo,  astronomers  roval  to  one 
of  the  ancient  emperors  of  that  people.  It  was  part  of  the 
duty  of  tliese  men  to  carefully  study  the  heavenly  movements, 
and  give  timely  warning  of  the  ap])roach  of  an  eclipse  or  other 
remarkable  phenomenon.  But,  neglecting  this  duty,  they  gave 
themselves  ud  to  drunkenness  and  notous  livinc:.  In  conse- 
quenGe,an  eclipse  of  the  sun  occurred  without  any  notice  being 
given ;  the  religious  rites  due  in  such  a  case  were  not  performed, 
and  China  was  exposed  to  the  anger  of  the  gods.  To  appease 
their  wrath,  the  unworthy  astronomers  were  seized  and  sum- 
marily executed  by  royal  command.      Some  historians  have 


INTRODUCTION.  3 

gone  so  far  as  to  fix  tho  date  of  this  occurrence,  which  is  vari- 
ously placed  at  from  2128  to  2159  yeai*s  before  the  Christian 
era.  If  this  is  correct,  it  is  the  earliest  of  which  profane  his- 
tory has  left  us  any  record. 

In  the  Hindoo  astronomy  we  see  the  peculiarities  of  the 
contemplative  Hindoo  mind  strongly  reflected.  Here  the 
imagination  revels  in  periods  of  time  which,  by  comparison, 
dwarf  even  the  measures  of  the  celestial  spaces  made  by  mod- 
ern astronomers.  In  this,  and  in  perhaps  other  ancient  sys- 
tems, we  find  references  to  a  su])posed  conjunction  of  all  the 
planets  3102  years  before  the  Christian  era.  Although  we 
have  every  reason  for  believing  that  this  conjunction  was 
learned,  not  from  any  actual  record  of  it,  but  by  calculating 
back  the  position  of  the  planets,  yet  the  very  fact  that  they 
were  able  to  make  this  calculation  shows  that  the  motions  of 
the  planets  must  have  been  observed  and  recorded  during 
many  generations,  either  by  the  Hindoos  themselves,  or  some 
other  people  from  w^hom  they  acquired  their  knowledge.  As 
a  matter  of  fact,  we  now  know  from  our  modern  tables  that 
this  conjunction  was  very  far  from  being  exact;  but  its  error 
could  not  be  certainly  detected  by  the  rude  observations  of  the 
times  in  question. 

Among  a  people  so  prone  as  the  ancient  Greeks  to  speculate 
upon  the  origin  and  nature  of  things,  while  neglecting  the  ob- 
servation of  natural  phenomena,  we  cannot  exjx;ct  to  find  any- 
thing that  can  be  considered  a  system  of  astronomy.  But  there 
are  some  ideas  attributed  to  Pythagoras  which  are  so  frequent- 
ly alluded  to,  and  so  closely  connected  witii  the  astronomy  of 
a  subsequent  age,  that  we  may  give  them  a  passing  mention. 
He  is  said  to  have  taught  that  the  heavenly  bodies  were  set 
in  a  number  of  crystalline  spheres,  in  the  common  centre  of 
which  the  earth  was  placed.  In  the  outer  of  these  spheres 
were  set  the  thousatids  of  fixed  stars  which  stud  the  firma- 
ment, while  each  of  tlie  seven  planets  luid  its  own  sphere.  The 
transparency  of  each  crystal  sphere  was  perfect,  so  that  the 
bodies  set  in  each  of  the  outer  spheres  were  visible  through 
all  the  inner  ones.     Tliese  spheres  all  rolled  round  on  each 


4  SYSTEM  OF  THE  WORLD  HISTORICALLY  DEVELOPED. 

otlicr  in  a  daily  revolution,  thus  causin<,^  the  rising  and  setting 
of  the  heavenly  bodies.  Tliis  rolling  of  the  spheres  on  each 
other  nmde  a  celestial  music,  the  "music  of  the  spheres," 
which  tilled  the  firmament,  but  was  of  too  elevated  a  char- 
acter to  be  heard  by  the  cars  of  mortals. 

It  must  be  admitted  that  the  idea  of  the  stars  being  set  in  a 
hollow  sphere  of  crystal,  forming  the  vault  of  the  firmament, 
was  a  very  natural  one.  They  seemed  to  revolve  around  the 
earth  every  day,  for  generation  after  generation,  without  the 
slightest  change  in  their  relative  positions.  If  there  were  no 
solid  connection  between  them,  it  does  not  seem  possible  that 
a  thousand  bodies  could  move  around  their  vast  circuit  for 
such  long  periods  of  time  without  a  single  one  of  them  vary- 
ing its  distance  from  one  of  the  others.  It  is  especially  diffi- 
cult to  conceive  how  they  could  all  move  around  the  same 
axis.  But  when  they  are  all  set  in  a  solid  sphere,  every  one  is 
made  secure  in  its  place.  The  planets  could  not  be  set  in  the 
same  sphere,  because  they  change  their  positions  among  the 
stars.  This  idea  of  the  sphericity  of  the  heavens  held  on  to 
the  minds  of  men  with  remarkable  tenacity.  The  funda- 
mental proposition  of  the  system,  both  of  Ptolemy  and  Coper- 
nicus, was  that  the  universe  is  spherical,  the  latter  seeking  to 
prove  the  naturalness  of  the  spherical  form  by  the  analogy 
of  a  drop  of  water,  although  the  theory  served  him  no  pur- 
pose whatever.  Faint  traces  of  the  idea  are  seen  here  and 
there  in  Kepler,  with  whom  it  vanished  from  the  mind  of  the 
race,  as  the  image  of  Santa  Claus  disappears  from  the  mind  of 
the  growing  child. 

Pythagoras  is  also  said  to  Inve  taught  in  his  esoteric  lect- 
ures that  the  sun  was  the  renl  centre  of  the  celestial  move- 
ments, and  that  the  earth  and  planets  moved  around  it,  and  it 
is  this  anticipation  of  the  Copernican  system  which  constitutes 
his  greatest  glory.  But  he  never  thought  proper  to  make  a 
public  avowal  of  this  doctrine,  and  even  presented  it  to  his 
disciples  somewhat  in  the  form  of  an  hypothesis.  It  must 
also  be  admitted  that  the  accounts  of  his  svstem  which  have 
reached  us  are  so  vague  and  so  filled  w  ith  metaphysical  specu- 


INTIiODUCTIOX.  5 

lation  that  it  is  questionable  whether  the  frequent  application 
of  his  name  to  the  modern  system  is  not  more  pedantic  than 
justifiable. 

The  Greek  astronomers  of  a  later  age  not  only  rejected  the 
vague  speculations  of  their  ancestors,  but  proved  themselves 
the  most  careful  observers  of  their  time,  and  lirst  made  astron- 
omy worthy  the  name  of  a  science.  From  this  Greek  astrono- 
my the  astronomy  of  our  own  time  may  be  considered  as  com- 
ing by  direct  descent.  Still,  were  it  not  for  the  absence  of  his- 
toric records,  we  could  probably  trace  back  both  their  theories 
and  their  system  of  observation  to  the  plains  of  Chaldea.  The 
zodiac  was  mapped  out  and  the  constellations  named  many 
centuries  before  they  commenced  their  observations,  and  these 
works  marked  quite  an  advanced  stage  of  development.  This 
prehistoric  knowledge  is,  however,  to  be  treated  hy  the  histo- 
rian rather  than  the  astronomer.  If  we  confine  ourselves  to 
men  whose  names  and  whose  labors  have  come  down  to  us, 
we  must  concede  to  Ilipparchus  the  honor  of  being  the  father 
of  astronomy.  Not  only  do  his  observations  of  the  heavenly 
bodies  appear  to  have  been  far  more  accurate  than  those  of 
any  of  his  predecessors,  but  he  also  determined  the  laws  of  the 
apparent  motions  of  the  planets,  and  prepared  tables  by  which 
these  motions  could  be  calculated.  Probably  he  was  the  first 
propounder  of  the  theory  of  epicyclic  motions  of  the  planets, 
commonly  called  after  the  name  of  his  successor,  Ptolemy,  who 
lived  three  centuries  later. 

Commencing  with  the  time  of  Ilipparchus,  the  general 
theory  of  the  structure  of  the  universe,  or  "sj'stem  of  the 
world,"  as  it  is  frequently  called,  exhibits  three  great  stages  of 
development,  each  stage  being  marked  by  a  system  quite  dif- 
ferent from  the  other  two  in  its  fundamental  principles.  These 
are: 

1.  The  so-called  Ptolemaic  system,  which,  however,  really 
belongs  to  Ilipparchus,  or  some  more  ancient  astronomer.  In 
this  system  the  motion  of  the  earth  is  ignored,  and  the  appar- 
ent motions  of  the  stars  and  planets  around  it  are  all  regarded 
as  real. 


6  SYSTEM  OF  THE   WOULD  UlSTOlilCALLY  DEVELOPED. 

2.  Tlie  Coperniean  system,  in  wlilcili  it  is  shown  that  the  sun 
is  really  the  centre  of  the  planetary  motions,  and  that  the  earth 
is  itself  a  planet,  both  turning  on  its  axis  and  revolving  round 
the  sun, 

3.  The  Newtonian  system,  in  which  all  the  celestial  motions 
are  explained  by  the  one  law  of  universal  gravitation. 

This  natural  order  of  development  shows  the  order  in  which 
a  knowledge  of  the  structure  of  the  universe  can  be  most 
clearly  pi-esented  to  the  mind  of  the  general  reader.  Wc 
shall  therefore  explain  this  structure  historically,  devoting  a 
separate  chapter  to  each  of  the  three  stages  of  development 
which  we  have  described.  We  commence  with  what  is  well 
known,  or,  at  least,  easily  seen  by  every  one  who  will  look  at 
the  heavens  with  sufhcient  care.  "\Vc  imagine  the  observer 
out-of-doors  on  a  starlit  night,  and  show  him  how  the  heav- 
enly bodies  seem  to  move  from  hour  to  hour.  Then,  we  show 
him  what  changes  he  will  see  in  their  aspects  if  he  contin- 
ues his  watch  through  nu)nths  and  years.  By  combining  the 
apparent  motions  thus  learned,  he  forms  for  himself  the  an- 
cient, or  Ptolemaic,  system  of  the  world.  Ila-'ing  this  system 
clearly  in  mind,  the  passage  to  that  of  Copernicus  is  but  a 
step.  It  consists  only  in  sliowing  that  certain  singular  oscilla- 
tions M'hich  the  sun  and  planets  seem  to  have  in  common  are 
really  due  to  a  revolution  of  the  earth  around  the  sun,  and 
that  the  apparent  daily  revolution  of  the  celestial  sphere  arises 
from  a  rotation  of  the  earth  on  its  own  axis.  The  laws  of 
the  true  motions  of  the  planets  being  perfected  by  Kepler, 
they  are  shown  by  Newton  to  be  included  in  the  one  law  of 
gravitation  towards  the  sun.  Such  is  the  course  of  thought  to 
which  we  first  invite  the  reader. 


THE  CELESTIAL  SPHERE. 


CHAPTER   I. 

THE    ANCIENT    ASTRONOMY,  OR   THE    APPARENT   MOTIONS   OF    THE 

HEAVENLY    BODIES. 

§  1.  The  Celestial  Flphere. 

It  is  a  fact  with  which  wo  arc  familiar  from  infancy,  that 
all  the  heavenly  bodies — sun,  moon,  and  stars — seem  to  be  set 
in  an  azure  vanlt,  which,  rising  liigh  over  our  heads,  curves 
down  to  the  horizon  on  every  side.  Here  the  earth,  on  which 
it  seems  to  rest,  prevents  our  tracing  it  farther.  Ijut  if  the 
earth  were  out  of  tlie  way,  or  were  perfectly  transparent,  we 
could  trace  tlio  vault  downwards  on  every  side  to  the  })oiiit 
beneath  our  feet,  and  could  see  sun,  moon,  and  stars  in  every 
direction.  The  celestial  vault  above  us,  with  the  correspond- 
ing one  below  us,  would  then  form  a  complete  sphere,  in  the 
centre  of  which  the  observer  would  seem  to  be  placed.  Tliis 
has  been  known  in  all  ages  as  the  celestial  sphere.  The  direc- 
tions or  apparent  positions  of  the  heavenly  bodies,  as  well  as 
their  apparent  motions,  have  always  been  defined  by  their  sit- 
uation and  motions  on  this  sphere.  The  fact  that  it  is  purely 
imaginary  does  not  diminish  its  value  as  enabling  us  to  form 
distinct  ideas  of  the  directions  of  the  heavenly  bodies  from  us. 

It  matters  not  how  large  we  suppose  this  sphere,  so  long  as 
we  always  suppose  the  observer  to  be  in  the  centre  of  it,  so 
that  it  shall  surround  him  on  all  sides  at  an  equal  distance. 
But  in  the  language  and  reasoning  of  exact  astronomy  it  is 
always  supposed  to  be  infinite,  as  then  the  observer  may  con- 
ceive of  himself  as  transported  to  any  other  point,  even  to  one 
of  the  heavenly  bodies  themselves,  and  still  be,  for  all  practical 
purposes,  in  the  centre  of  the  sphere.  In  this  case,  however, 
the  heavenly  bodies  are  not  considered  as  attached  to  the  cir- 


8 


SYSTEM  OF  THE   WORLD  HISTORICALLY  DEVELOPED. 


cumfercnce  of  tlie  infinite  sphere,  but  only  as  lying  on  the  line 
of  sight  extending  from  the  observer  to  some  point  of  the 
sphere.  Their  relation  to  it  may  be  easily  understood  by  the 
observer  conceiving  himself  to  be  luminous,  and  to  throw  out 
rays  in  every  directio?i  to  tlie  infinitely  distant  sphjre.  Then 
the  apparent  positions  of  the  various  heavenly  bodies  will  be 
those  in  which  their  shadows  strike  the  sphere.  For  instance, 
the  observer  standing;  on  the  earth  and  looking  at  the  moon, 


Fio.  1 Section  of  the  imaginary  celestial  sphere.    The  obperver  at  0,  lookiup;  at  the 

stars  or  other  bodies,  marked  p,  q,  r,  s,  t,  n,  v,  will  imagine  them  situated  at  P,  Q,  li,  S, 
T,  U,  r,  on  tlie  surface  of  the  sphere,  where  they  will  appear  projected  along  the 
straight  j>P,5Q,  etc. 

the  shadow  of  the  latter  will  strike  the  sphere  at  a  point  on  a 
straight  line  drawn  from  the  observer's  eye  through  the  centre 
of  the  moon,  and  continued  till  it  meets  the  sphere.  The  point 
of  meeting  will  represent  the  position  of  the  moon  as  seen  by 
the  observer.  Now,  suppose  the  latter  transported  to  the  moon. 
Then,  looking  back  at  the  earth,  he  will  see  it  projected  on  the 
S}>liere  in  a  point  diametrically  ojiposite  to  that  in  which  he 
formerly  saw'  the  moon.     To  whatever  planet  he  might  trans- 


TEE  DIUBNAL  MOTION.  9 

port  himself,  he  would  sec  the  earth  and  the  other  planets  pro- 
jected on  this  imaginary  sphere  precisely  as  we  always  seem 
to  see  the  heavenly  bodies  so  projected. 

This  is  all  that  is  left  of  the  old  crystalline  spheres  of  Py- 
thagoras by  modern  astronomy.  From  being  a  solid  which 
held  all  the  stars,  the  sphere  has  become  entirely  immaterial, 
a  mere  conception  of  the  mind,  to  enable  it  to  define  the  di- 
rections in  which  the  heavenly  bodies  are  seen.  By  examin- 
ing the  figure  it  vvill  be  clear  that  all  bodies  which  lie  in  the 
same  straight  line  from  the  observer  will  appear  on  the  same 
point  of  the  sphere.  For  instance,  bodies  at  the  three  points 
marked  t  will  all  be  seen  as  if  they  were  at  T. 

§  2.  The  Diurnal  Motion. 

If  we  watch  the  heavenly  bodies  for  a  few  hours  we  shall 
always  find  them  in  motion,  those  in  the  east  rising  upwards, 
those  in  the  south  moving  towards  the  west,  and  those  in  the 
west  sinking  below  the  horizon.  We  know  that  this  motion 
is  only  apparent,  arising  from  the  rotation  of  the  earth  on  its 
axis ;  but  as  we  wish,  in  this  chapter,  only  to  describe  things 
as  they  appear,  we  may  speak  of  the  motion  as  real.  A  few 
days'  watching  will  show  that  the  whole  celestial  sphere  seems 
to  revolve,  as  on  an  axis,  every  day.  It  is  to  this  revolution, 
carrying  the  sun  alternately  above  and  below  the  horizon,  that 
the  alternations  of  day  and  night  are  due.  The  nature  and 
effects  of  this  motion  can  best  be  studied  by  watching  tlie  ap- 
parent movement  of  the  stai*s  at  night.  We  should  soon  learn 
from  such  a  watch  that  there  is  one  point  in  the  heavens,  or 
on  the  celestial  sphere,  which  does  not  move  at  all.  In  our 
latitudes  this  point  is  situated  in  the  north,  between  the  zenith 
and  the  horizon,  and  is  called  the  pole.  Around  this  pole,  as 
a  fixed  centre,  all  the  heavenly  bodies  seem  to  revolve,  each 
one  moving  in  a  circle,  the  size  of  which  depends  on  the  dis- 
tance of  the  body  from  tlie  pole.  There  is  no  star  situated 
exactly  at  the  pole,  but  there  is  one  which,  being  situated  lit- 
tle more  than  a  degree  distant,  describes  so  small  a  circle  that 
the  unaided  eye  cannot  see  any  change  of  place  without  mak- 


10        SYSTEM  OF  THE  WORLD  HISTORICALLY  DEVELOPED. 

ing  some  exact  and  careful  observation.  This  is  therefore 
called  the  pole  star.  The  pole  star  can  nearly  always  be  very 
readily  found  by  means  of  the  pointers,  two  stai-s  of  the  con- 
stellation Ursa  3fajor,  the  Great  Bear,  or,  as  it  is  familiarly 
called,  the  Dipper.  By  referring  to  the  figure,  the  reader  will 
readily  find  this  constellation,  by  the  dotted  line  from  the  pole 
and  thence  the  pole  star,  which  is  near  the  centre  of  the  map. 


Fig.  2.— Map  of  the  priiicipul  stars  of  the  northern  sky,  showing  the  constellations  which 
never  set  iu  latitude  40°,  but  revolve  round  the  pole  star  every  day  in  the  direction 
shown  by  the  arrows.  The  two  h)wer  stars  of  Urna  Major,  on  the  left  cf  iue  map, 
point  to  the  pole  star  in  the  centre. 

The  altitude  of  the  pole  is  equal  to  the  latitude  of  the  place. 
In  the  Middle  States  the  latiti^de  is  generally  not  far  from 
forty  degrees;  the  pole  is  therefore  a  little  nearer  to  the  hori- 
zon than  to  the  zenith.  In  Maine  and  Canada  it  is  about  half- 
way between  these  points,  while  in  England  and  Northern 
Europe  it  is  nearer  the  zenith. 


THE  DIURNAL  MOTION.  11 

Now,  to  see  the  effect  of  the  diunial  motion  near  the  pole, 
let  us  watch  any  star  in  the  north  between  the  pole  and  the 
horizon.  We  shall  soon  see  that,  instead  of  moving  from  east 
to  west,  as  we  are  accustomed  to  see  the  heavenly  bodies  move, 
it  really  moves  towards  the  east.  After  passing  the  north 
point,  it  begins  to  curve  its  course  upwards,  until,  in  the  north- 
east, its  motion  is  vertical.  Then  it  turns  gradually  to  the 
west,  passing  as  far  above  the  pole  as  it  did  below  it,  and,  sink- 
ing down  on  the  west  of  the  pole,  it  again  passes  under  it. 
The  passage  above  the  pole  is  called  tlie  upper  culmination, 
and  that  below  it  the  lower  one.  The  course  around  the  pole 
is  shown  by  the  arrows  on  Fig.  2.  We  cannot  with  the  naked 
eye  follow  it  all  the  way  round,  on  account  of  the  intervention 
of  daylight ;  but  by  continuing  our  watch  every  clear  night  for 
a  year,  we  should  see  it  in  every  point  of  its  course.  A  star 
following  the  course  we  have  described  never  sets,  but  may  be 
seen  every  clear  night.  If  we  imagine  a  circle  drawn  round 
the  pole  at  such  a  distance  as  just  to  touch  the  horizon,  all  the 
stars  situated  within  this  circle  will  move  in  this  way ;  this  is 
therefore  called  the  circle  of  perpetual  apparition. 

As  we  go  away  from  the  pole  we  shall  find  the  stars  mov- 
ing in  larger  circles,  passing  higher  up  over  the  pole,  and  lower 
down  belovV  it,  until  we  reach  the  circle  of  perpetual  appari- 
tion, when  they  will  just  graze  the  horizon.  Outside  this  circle 
every  star  nnist  dip  below  the  horizon  for  a  greater  or  less 
time,  depending  on  its  distance.  If  it  be  only  a  few  degrees 
outside,  it  will  set  in  the  north-west,  or  between  north  and 
north-west ;  and,  after  a  few  hours  only,  it  will  be  seen  to  rife 
again  between  north  and  north-east,  Ihv  ing  done  little  more 
than  graze  the  horizon.  The  possibility  of  a  body  rising  so 
soon  after  having  set  does  not  always  occur  to  those  who  live 
in  moderate  latitudes.  In  July,  1874,  Coggia's  comet  set  in 
the  north-west  about  nine  o'clock  in  the  evening,  and  rose 
again  about  three  o'clock  in  the  morning;  and  some  intelligent 
people  who  then  saw  it  east  of  the  pole  supposed  it  could  not 
be  the  same  one  that  had  set  the  evening  before. 

Passing  outside  the  circle  of  perpetual  apparition,  we  find 


12       SYSTEM  OF  THE  WORLD  HISTOBICALLY  DEVELOPED. 

that  the  stars  pass  soutli  of  tlie  zenitli  at  their  upper  cuhnina- 
tion,  that  they  set  more  quickly,  and  that  they  are  a  longer 
time  below  the  horizon.  This  may  be  seen  in  Fig.  3,  the  por- 
tion of  the  sphere  to  which  we  refer  being  between  the  celes- 
tial equator  and  the  line  LJ^.  AVhen  we  reach  the  equator 
one-half  the  course  will  be  above  and  one-half  beiow  the  liori- 


Fio.  3.— The  celestial  sphere  and  diurnal  motion.  S  is  tlie  soutii  horizon,  X  the  north  hori- 
zon, Z  the  zenith.  The  circle  L.V  around  the  north  pole  contains  the  stars  shown  in 
Fig.  2 ;  and  the  observer  at  O,  in  tiie  centre  of  the  sphere,  looking  to  the  north,  sees  the 
etars  as  they  are  depicted  in  that  figure.  The  arrows  show  the  direction  of  the  diurnal 
motion  in  the  west. 

zon.  South  of  the  equator  the  circles  described  by  the  stars 
become  smaller  once  more,  and  more  than  half  their  course  is 
below  the  horizon.  Near  the  south  horizon  the  stars  only  shuw 
themselves  above  the  horizon  for  a  short  time,  while  below  it 
there  is  a  circle  of  perpetual  disappearan-e,  the  stars  in  which, 
to  us,  never  rise  at  all.     This  circle  is  of  the  same  magnitude 


MOTION  OF  THE  SUN  AMONG   L.IE  STA{:S.  13 

with  that  of  perpetual  apparition,  and  the  south  pole  is  situated 
in  its  centre,  just  as  the  north  pole  is  in  the  centre  of  the  other. 

If  we  travel  southward  we  find  that  the  north  pole  gradually 
sinks  towards  the  horizon,  while  new  stars  come  into  view  above 
the  south  horizon ;  consequently  the  circles  of  perpetual  appari- 
tion and  of  perpetual  disappearance  both  grow  smaller.  When 
we  reach  the  earth's  equator  the  south  pole  has  risen  to  the 
south  horizon,  the  north  pole  has  sunk  to  the  north  hori- 
zon ;  the  celestial  equator  passes  from  east  to  west  directly 
overhead ;  and  all  the  heavenly  bodies  in  their  diurnal  revolu- 
tions describe  circles  of  which  one  half  is  above  and  the  other 
half  below  the  horizon.     These  circles  are  all  vertical. 

South  of  the  equator  only  the  south  pole  is  visible,  the  north 
one,  which  we  see,  being  now  below  the  horizon.  Beyond  the 
southern  tropic  the  sun  is  north  at  noon,  and,  instead  of  mov- 
ing from  left  to  right,  its  course  is  from  right  to  left. 

The  laws  of  the  diurnal  motion  which  we  have  described 
may  be  summed  up  as  follows : 

1.  The  celestial  sphere,  with  the  sun,  moon,  and  stars,  seems 
to  revolve  daily  around  an  inclined  axis  passing  through  the 
point  where  we  ma;;  chance  to  stand. 

2.  The  upper  end  of  this  axis  points  (in  this  hemisphere)  to 
the  north  pole ;  the  other  end  passes  into  the  earth,  and  points 
to  the  south  pole,  which  is  diametrically  opposite,  and  therefore 
below  the  horizon. 

3.  All  the  fixed  stars  during  this  revolution  move  together, 
keeping  at  the  same  distance  from  each  other,  as  if  the  revolv- 
ing celestial  sphere  were  solid,  and  they  were  set  in  it. 

4.  The  circle  drawn  round  the  heavens  half-way  between 
the  two  poles  being  the  celestial  equator,  all  bodies  north  of 
this  equator  perform  more  than  half  their  revolution  above 
the  horizon,  while  south  of  it  less  than  half  is  above  it. 

§  3.  Motion  of  the  Sun  among  the  Stars. 

The  most  obvious  classification  of  the  heavenly  bodies  which 
we  see  with  the  naked  eye  is  that  of  sun,  moon,  and  stars. 
But  there  is  also  this  difference  among  the  stare,  that  while  the 


14       SYSTEM  OF  THE  WOULD  HISTORICALLY  DEVELOPED. 

great  mass  of  them  preserve  the  same  relative  position  on  the 
celestial  sphere,  year  after  year  and  century  after  century,  there 
are  five  which  constantly  change  their  positions  relatively  to 
the  others.  Their  names  are  Mercury,  Venus,  Mars,  Jupiter, 
and  Saturn.  These  five,  M-ith  the  sun  and  moon,  constitute  the 
seven  planets,  or  wandering  stars,  of  the  ancients,  the  motions 
of  which  are  next  to  be  described.  Taking  out  the  seven 
planets,  the  remaining  heavenly  bodies  visible  to  the  naked 
eye  are  termed  the  Fixed  Stars,  because  they  have  no  appar- 
ent motion,  except  the  regular  diuiiial  revolution  described  in 
the  last  section.  But  if  we  note  tJie  positions  of  the  sun, 
moon,  and  planets  among  the  stars  for  a  number  of  successive 
nights,  we  shall  lind  certain  slow  changes  among  them  which 
we  shall  now  describe,  beginning  with  the  sun.  In  studying 
this  description,  the  reader  must  remember  that  we  are  not 
seeking  for  the  apparent  diurnal  motion,  but  only  certain 
much  slower  motions  of  the  planets  relative  to  the  fixed  stars, 
such  as  would  be  seen  if  the  earth  did  not  rotate  on  its  axis. 

If  we  observe,  night  .after  night,  the  exact  hour  and  minute 
at  which  a  star  passes  anj'  point  by  its  diurnal  revolution,  we 
shall  find  that  passage  to  occur  some  four  minutes  earlier 
every  evening  than  it  did  the  evening  before.  The  starry 
sphere  therefore  revolves,  not  in  24  hours,  but  in  23  hours 
56  minutes.  In  consequence,  if  we  note  its  position  at  the 
same  hour  night  after  night,  we  shall  find  it  to  be  farther  and 
farther  to  the  west.  Let  us  take,  for  example,  the  brightest 
star  in  the  constellation  Leo,  represented  on  Map  III.,  and 
commonly  known  as  Regulus.  If  we  watch  it  on  tlie  22d  of 
March,  we  shall  find  that  it  passes  the  meridian  at  ten  o'clock 
in  the  evening.  On  April  22d  it  passes  at  eight  o'clock,  and 
at  ten  it  is  two  hours  west  of  the  meridian.  On  the  same  dav 
of  May  it  passes  at  six,  before  sunset,  so  that  it  cannot  be  seen 
on  the  meridian  at  all.  When  it  first  becomes  visible  in  the 
evening  twilight,  it  will  be  an  hour  or  more  west  of  the  me- 
ridian. In  June  it  will  be  three  hours  west,  and  by  the  end  of 
July  it  will  set  during  twilight,  and  will  soon  be  entirely  lost 
in  the  rays  of  the  sun.     This  shows  that  during  the  months  in 


MOTION  OF  THE  SUN  AMONG  THE  STAES.  15 

question  the  sun  has  been  approaching  the  star  from  the  west, 
and  in  August  has  got  so  near  it  that  it  is  no  longer  visible. 

Carrying  forward  our  computation,  we  find  that  on  August 
21st  the  stpr  crosses  the  meridian  at  noon,  and  therefore  at 
nearly  the  same  time  with  the  sun.  In  September  it  crosses 
at  ten  in  the  morning,  while  the  sun  is  on  the  eastern  side. 
The  sun  has  therefore  passed  from  the  west  to  the  east  of  the 
star,  and  the  latter  can  be  seen  rising  in  the  morning  twilight 
before  the  sun.  It  constantly  rises  earlier  and  earlier,  and 
therefore  farther  from  the  sun,  until  February,  when  it  rises 
at  sunset  and  sets  at  sunrise ;  and  is  therefore  directly  opposite 
the  sun.  In  March  the  star  would  cross  the  meridian  at  ten 
o'clock  once  more,  showing  that  in  the  course  of  a  year  the 
sun  and  star  had  resumed  their  first  position.  But,  wliile  the 
sun  has  risen  and  set  365  times,  the  star  has  risen  and  set  366 
times,  the  sun  having  lost  an  entire  revolution  by  the  slow 
backward  motion  we  have  described. 

If  the  stars  were  visil)le  in  the  daytime  (as  they  would  be 
but  for  the  atmospliere),  the  apparent  motion  of  the  sun  among 
them  could  be  seen  in  the  course  of  a  single  day.  For  in- 
stance, if  we  could  have  seen  Regulus  rise  on  the  morning  of 
August  20th,  1876,  we  should  have  seen  the  sun  a  little  south 
and  west  of  it,  the  relative  position  of  the  sun  being  as  shown 
by  the  circle  numbered  1  in  the  figure.  ... 

Watching  the  star  all  day,  we  should  find         OOOO 
that  at  sunset  it  was  north  from  the  sun,  4    s    2    i 

n  •      1       AT        o         rpi  i  j    Fio.  4.— Motion  of  the  sun 

as  from  circle  No.  2.  The  sun  ^  would  j,,,^  ^^^^  ,^,,  jj,^,„,^^ 
during  the  day  have  moved  nearly  its  own  "i^o»t  August  aeth  of 
diameter.  Next  morning  we  should  have  ^^^"^^ 
seen  that  the  sun  had  gone  past  the  star  into  position  3,  so 
that  the  latter  would  now  rise  before  the  former.  By  sun- 
set it  would  have  advanced  to  position  4,  and  so  forth.  The 
path  which  the  sun  describes  among  the  stars  in  his  annual 
revolution  is  called  the  ecliptic.  It  is  marked  down  on  Maps 
II.,  III.,  IV.,  and  v.,  and  the  months  in  which  the  sun  passes 
througli  each  portion  of  the  ecliptic  are  also  indicated.  A 
belt  of  the  heavens,  extending  a  few  degrees  on  each  side  of 


IG        SYSTEM  OF  THE  WORLD  UISTOIUCALLY  DEVELOPED. 

the  ecliptic,  is  called  the  zodiac.  The  poles  of  the  ecliptic  are 
two  opposite  points,  each  in  the  centre  of  one  of  the  two  hemi- 
spheres into  which  the  ecliptic  divides  the  celestial  sphere. 

The  determination  of  the  solar  motion  around  the  ecliptic 
may  be  considered  the  birth  of  astronomical  science.  The 
prehistoric  astronomers  divided  the  eclii>tic  and  zodiac  into 
jtwehe  parts,  now  familiarly  known  as  tlie  signs  of  the  zodiac. 
This  proceeding  was  probably  suggested  by  the  needs  of  agri- 
culture, and  of  the  chronological  reckoning  of  years.  A  very 
little  observation  would  show  that  the  changes  of  the  seasons 
are  due  to  the  variations  in  the  meridian  altitude  of  the  sun, 
and  in  the  length  of  the  day ;  but  it  was  only  by  a  careful 
study  of  the  position  of  tlie  ecliptic,  and  the  motion  of  the  sun 
in  it,  that  it  could  be  learned  how  these  variations  in  the  daily 
course  of  the  sun  were  brought  about.  This  study  showed 
that  they  were  due  to  the  fact  that  the  ecliptic  and  e(piator 
did  not  coincide,  but  were  inclined  to  each  other  at  an  angle 
of  between  twenty-three  and  twenty-four  degrees.  This  in- 
clination is  known  as  t\\2  obliquity  of  the  ecliptic.  The  two 
circles,  equator  and  ecliptic,  cross  each  other  at  two  opposite 
points,  the  positions  of  which  among  the  stars  may  be  seen  by 
reference  to  Maps  II. -Y.  When  the  sun  is  at  either  of 
these  points,  it  rises  exactly  in  tlie  east,  and  sets  exactly  in  the 
west;  one-half  its  diurnal  course  is  above  the  horizon,  and  the 
other  half  below.  Tiie  days  and  nights  are  therefore  of  equal 
length,  from  which  the  two  points  in  question  are  called  the 
£^guinoxes. 

The  vernal  equinox  is  en  the  right-hand  edge  of  Map  II. 
Leaving  that  equinox  about  March  21st,  the  sun  crosses  over 
the  region  represented  by  the  map  in  the  course  of  the  next 
three  months,  working  northward  as  it  does  so,  until  June  20th, 
when  it  is  on  the  left-hand  edge  of  the  map,  23|°  north  of  the 
equatoi*.  This  point  of  the  ecliptic  is  called  the  summer  solstice, 
being  that  in  which  the  sun  attains  its  greatest  northern  declina- 
tion. When  near  this  solstice,  it  rises  north  of  east,  culmi- 
nates at  a  high  altitude  (in  our  latitudes),  and  sets  north  of 
west.     As  explained  in  describing  the  diurnal  motion  of  an 


MOTION  OF  THE  SUN  AMONG   THE  STARS.  17 

object  north  of  tlie  celestial  equator,  more  than  half  the  daily 
course  of  the  sun  is  now  above  our  horizon,  so  that  our  days 
are  longer  than  our  nights,  while  the  great  meridian  altitude 
of  the  sun  prod\ices  the  heats  of  summer. 

The  portion  of  the  ecliptic  represented  on  Map  II.,  com- 
mencing at  the  vernal  equinox,  where  the  sun  crosses  the  equa- 
tor, was  divided  by  the  early  astronomers  into  the  three  signs 
of  Aries,  the  Ram  ;  Taurus,  the  Bull ;  and  Gemini,  the  Twins. 
It  will  be  seen  that  these  signs  no  longer  coincide  with  the 
constellations  of  the  same  name :  ■  s  is  owing  to  a  change  in 
the  position  of  the  equator,  which  will  be  described  presently. 

Turning  to  Map  III.,  we  see  that  during  the  three  months, 
from  June  to  September,  the  sun  works  downwards  towards 
the  equator,  reaching  it  about  September  20th.  The  point  of 
crossing  marks  the  autumnal  equinox,  found  also  on  the  right 
hand  of  Map  IV.  The  days  and  nights  are  now  once  more  of 
equal  length. 

During  the  next  six  months  the  sun  is  passing  over  the  re- 
gions represented  on  Maps  IV.  and  V.,  and  is  south  of  the 
equator,  its  greatest  southern  declination,  or  "the  southern 
solstice,"  being  reached  about  December  2l8t.  More  than 
half  its  daily  course  is  then  below  the  horizon,  so  that  in  our 
latitudes  the  nights  are  longer  tlian  the  days,  and  the  low 
noonday  altitude  of  the  sun  gives  rise  to  the  colds  of  winter. 

We  have  no  historic  record  of  this  division  of  the  zodiac 
into  signs,  and  the  ideas  of  the  authors  can  only  be  inferred 
from  collateral  circumstances.  It  has  been  fancied  that  the 
names  were  suggested  by  the  seasons,  the  agricultural  opera- 
tions, and  so  on.  Thus  the  spring  signs  (Aries,  the  Ram :  Tau- 
rus, the  Bull;  and  Gemini,  the  Twins  are  supposed  to  mark  the 
bringing  forth  of  young  by  the  flocks  and  herds.  Cancer,  the 
Crab,  marks  the  time  when  the  sun,  having  attained  its  great- 
est declination,  begins  to  go  back  towards  the  equator;  and  the 
crab  having  been  supposed  to  move  backwards,  his  name  was 
given  to  this  sign.  Leo,  the  Lion,  symbolizes  the  fierce  heat 
of  summer;  and  Virgo,  the  Virgin,  gleaning  corn,  symbolizes 
the  harvest.     In  Libra,  the  Balance,  the  day  and  night  balance 

3 


18        SYSTEM  OF  THE  WOULD  HISTORICALLY  DEVELOPED. 

eacli  otlicr,  being  of  equal  lenpjtli.     Scorpio,  the  Scorpion,  is 

supposed  to  liave  marked  the  presence  of  veuoinons  reptiles  in 

October;  M'hile  Sai^ittarius,  the  Archer,  symbolizes  the  season 

of  liuutiug.     The  explanation  of  Capricornus,  the  Goat,  is  more 

fanciful,  if  possible,  tiian  that  of  Cancer.     It  was  supposed  that 

this  animal,  ascending  the  hill  as  he  feeds,  in  order  to  reach 

the  grass  more  easily,  on  reaching  the  tof),  turns  back  again,  so 

that  his  name  was  used  to  mark  the  sign  in  which  the  sun, 

from  going  south,  begins  to  return  to  the  north.     Aquarius, 

the  Water-bearer,  symbolizes  the  winter  rains;  and  Pisces,  the 

Fishes,  tlie  season  of  fishes. 

All  this  is,  however,  mere  conjecture;  the  only  coincidences 

at  all  strikinj;  beinsr  Viry-o  and  Libra.     The  names  of  the  con- 
es o         o 

stellations  were  probably  given  to  them  several  centuries,  per- 
haps oven  thousands  of  years,  before  the  Christian  era ;  and  in 
that  case  the  zodiacal  constellations  would  not  have  correspond- 
ed to  the  seasons  we  have  indicated.  An  attempt  has  even  been 
made  to  show  that  the  names  of  the  zodiacal  constellations  were 
intended  to  commemorate  the  twelve  labors  of  Hercules;  but 
this  theory  rests  on  no  better  foundation  than  the  other. 

Tlie  zodiacal  constellations  occupy  quite  unequal  spaces  in 
the  heavens,  as  may  be  seen  by  inspection  of  the  maps.  In 
the  beginning  they  were  simply  twelve  houses  for  the  sun, 
which  that  luminary  occupied  in  the  course  of  the  year.  Ilip- 
parchus  found  this  system  entirely  insufficient  for  exact  astron- 
omy, and  therefore  divided  the  ecliptic  and  zodiac  into  twelve 
equal  parts,  of  30°  each,  called  signs  of  the  zodiac.  He  gave 
to  these  signs  the  names  of  the  constellations  most  nearly  cor- 
responding  to  them.  Commencing  at  the  vernal  equinox,  the 
first  arc  of  30°  was  called  the  sign  Aries,  the  second  the  sign 
Taurus,  and  so  forth.  The  mode  of  reckoning  positions  on 
the  ecliptic  by  signs  was  continued  until  the  last  centinw,  but 
is  no  longer  in  use  among  professional  astronomers,  owing  to 
its  inconvenience.  The  Avhole  ecliptic  is  now  divided  into 
360°,  like  any  other  circle,  the  count  conunencing  at  the  vernal 
equinox,  and  following  the  direction  of  the  sun's  motion  all  the 
way  round  to  300°. 


PRECESSION  OF  THE  EQUINOXES.  19 

§  4,  Precession  of  the  Equinoxes. 

By  comparing  his  own  observations  with  those  of  Mreceding 
astronomers,  Ilipparchus  found  that  tlie  equinoxes  wore  slowly 
sliifting  tlieir  places  among  the  stars,  the  change  being  at  least 
a  degree  in  a  century  towards  the  w  est.     His  successors  deter- 
mined it  with  greater  exactness,  and  it  is  now  known  to  be 
nearly  a  degree  in  seventy  years.    Careful  study  of  the  change 
shows  that  it  is  due  mainly  to  a  motion  of  the  equator,  which 
again  arises  from  a  change  in  the  direction  of  the  pole.     The 
position  of  the  ecliptic  among  the  stars  varies  so  slowly  that  the 
change  can  be  seen  only  by  the  refined  observations  of  modern 
times.    In  the  explanation  of  the  diurnal  motion,  it  was  stated 
that  there  was  a  certain  point  in  the  heavens  around  which  all 
the  heavenly  bodies  seem  to  perform  a  daily  revolution.     This 
point,  the  pole  of  tlie  heavens,  is  marked  on  the  centre  of  Map 
I.,  and  is  also  in  the  centre  of  Fig.  2,  page  10.    It  is  little  more 
than  a  degree  distant  from  the  pole  star.    Now,  precession  real- 
ly consists  in  a  very  slow  motion  of  this  pole  around  the  pole 
of  the  ecliptic,  the  rate  of  motion  being  such  as  to  carry  it  all 
the  way  round  in  about  25,300  years.      The  exact  time  has 
never  been  calculated,  and  would  not  always  be  the  same,  ow- 
ing to  some  small  variations  to  which  the  motion  is  subject; 
but  it  will  never  differ  much  from  this.    There  is  a  very  slight 
motion  to  the  ecliptic  itself,  and  therefore  to  its  pole ;  and  this 
fact  renders  the  motion  of  the  pole  of  the  equator  around  it 
somewhat  complicated ;  but  the  curve  described  by  the  latter 
is  very  nearly  a  circle  46°  in  diameter.     In  the  time  of  Ilip- 
parchus, our  present  pole  star  was  12°  from  the  pole.    The  pole 
has  been  approaching  it  steadily  ever  since,  and  will  continue 
to  approach  it  till  about  the  year  2100,  when  it  will  slowly 
pass  by  it  at  the  distance  of  less  than  half  a  degree.     The 
course  of  the  pole  during  the  next  12,000  years  is  laid  dowm 
on  the  map,  and  it  will  be  seen  that  at  the  end  of  that  time 
it  will  be  near  the  constellation  Lyra.     Since  the  equator  is 
always  90°  distant  from  the  pole,  there  will  be  a  correspond- 
ing motion  to  it,  and  hence  to  the  point  of  its  crossing  the 


'20       SYSTIiM  OF  TUE  WUIILI)  lIISTOIilCALLY  DEVELOPED. 

ecliptic.  To  show  this,  the  position  of  the  equator  2000  years 
ago,  as  well  as  its  present  position,  is  given  on  Map  II. 

The  reader  will,  of  course,  understand  that  the  various  ce- 
lestial movements  of  which  we  have  spoke"  'n  this  chapter  are 
only  ajjparent  motions,  and  are  due  to  the  motion  of  the  earth 
itself,  as  will  be  explained  in  the  chai)ter  on  the  Copernican 
system.  The  diurnal  revolution  of  the  celestial  sphere  is  due 
to  the  rotation  of  the  earth  on  its  axis,  while  precession  is  real- 
ly a  change  in  the  dii'cction  of  that  axis. 

One  important  effect  of  precession  is  that  one  revolution  of 
the  sun  among  the  stars  does  not  accurately  correspond  to  the 
return  of  the  same  seasons.  Tiie  latter  depend  upon  the  posi- 
tion of  the  sun  relative  to  the  equinox,  the  time  when  the  sun 
crosses  the  equator  towards  the  north  always  marking  the  sea- 
son of  spring  (in  the  northern  hemisphere),  no  matter  where 
the  sun  may  be  among  the  stars.  If  the  equator  did  not  move, 
the  sun  would  always  cross  it  at  nearly  the  same  point  among 
the  stars.  But  when,  starting  from  the  vernal  equinox,  it 
makes  the  circuit  of  the  heavens,  and  returns  to  it  again,  the 
motion  of  the  equator  has  been  such  that  the  sun  crosses  it 
20  minutes  before  it  reaches  the  same  star.  In  one  year,  this 
difference  is  very  small ;  but  by  its  constant  accumulation,  at 
the  rate  of  20  minutes  a  year,  it  becomes  very  considerable 
after  the  lapse  of  centuries.  We  must,  therefore,  distinguish 
between  the  sidereal  and  the  tropical  year,  the  former  being 
the  period  required  for  one  revolution  of  tiie  sun  among  the 
stars,  the  latter  that  required  for  his  return  to  the  same  equi- 
nox, whence  it  is  also  called  the  equinoctial  year.  Tlie  exact 
lengths  of  these  respective  years  are: 

Days.  Days.     Hours.    Min.      Sec. 

Sidereal  year 3G5.25G.'}G  =  3Gr.      G        •>       9 

Tropical  year 3Gr,. 24220  =  3Gr>       5      48     46 

Since  the  recurrence  of  tlie  seasons  depends  on  the  tropical 
year,  the  latter  is  the  one  to  be  used  in  forming  the  calendar, 
and  for  the  purposes  of  civil  life  generally.  Its  true  length  is 
11  minutes  14  seconds  less  than  365^  days.  Some  results  of 
this  difference  will  be  shown  in  explaining  the  calendar. 


THE  MOON.  21 

§  5.  The  Moon. 

Every  one  knows  that  the  moon  makes  a  revolution  in  the 
celestial  sphere  in  about  a  month,  and  that  during  its  revolu- 
tion it  presents  a  nuMiber  of  different  phases,  known  as  "  new 
moon,"  "first  quarter,"  "full  moon,"  and  s-  on,  depending 
on  its  position  relative  to  the  sun.  A  study  of  these  phases 
during  a  single  revoKition  will  make  it  clear  that  the  moon  is 
a  globular  dark  body,  illuminated  by  the  light  of  the  sun,  a 
fact  which  has  been  evident  to  careful  observers  from  the  re- 
motest antiquity.  This  may  be  illustrated  by  taking  a  lar<jje 
globe  to  reprofient  the  moon,  painting  one  half  white,  to  rep- 
resent the  ht,.^  on  which  the  sun  shines,  and  the  other  half 
dark.  Viewing  it  at  a  proper  distance,  and  turning  it  into 
different  positions,  it  will  be  found  that  the  visible  i)art  of  the 
white  half  may  be  made  to  imitate  the  various  appearances  of 
the  moon. 

As  the  sun  makes  a  revolution  around  the  celestial  sphere 
in  a  year,  so  the  moon  makes  a  similar  revolution  among  the 
stars  in  a  little  more  than  27  days.  This  motion  can  be  seen 
on  any  clear  night  between  first  quarter  and  full  moon,  'f  the 
moon  happens  to  be  near  a  bright  star.  If  the  ])osition  of  the 
moon  relatively  to  the  star  be  noted  from  hour  to  hour,  it  will 
be  found  that  she  is  constantly  working  towards  the  east  by  a 
distance  equal  to  her  own  diameter  in  an  hour.  The  follow- 
ing night  she  will  bo  found  from  12°  to  14°  east  of  the  star, 
and  will  rise,  cross  the  meridian,  and  set  from  half  an  hour  to 
an  hour  later  than  she  did  the  preceding  night.  At  the  end 
of  27  days  8  hours,  she  will  be  back  in  the  same  position 
among  the  stars  in  which  she  was  first  seen. 

If,  howevei",  starting  from  one  new  moon,  we  count  forwards 
this  period,  we  shall  find  that  the  moon,  although  she  has  re- 
turned to  the  same  position  among  the  stars,  has  not  got  back 
to  new  moon  again.  The  reason  is  that  the  sun  has  moved 
forwards,  in  virtue  of  his  apparent  annual  motion,  so  far  that 
it  will  require  more  than  two  days  for  the  moon  to  overtake 
him.     So,  although  the  moon  really  revolves  around  the  earth 


22       iSYSTHM  OF  THE   WORLD  HISTORICALLY  DEVELOPED. 

in  27i  days,  the  average  interval  between  one  new  moon  and 
the  next  is  20^  days. 

A  comparison  of  the  phases  of  the  moon  with  her  direction 
will  show  that  the  sun  is  many  times  more  distant  than  the 
moon.  In  Fig.  5,  let  E  be  the  position  of  an  observer  on  the 
earth,  Jf  the  moon,  and  S  the  sun,  illuminating  one  half  of  it. 
When  tlie  observer  sees  the  moon  in  her  first  quarter — that  is, 
when  her  disk  appears  exactly  half  illuminated — the  angle  at 


the  moon,  between  the  observer  and  the  sun,  must  be  a  right 
angle.  If  the  sun  were  only  about  four  times  as  far  as  the 
moon,  as  in  the  figure,  the  observer,  by  measuring  the  angle 
SEM  hiitweew  the  sun  and  moon,  would  tind  it  to  be  75°  ;  and 
the  nearer  the  sun,  the  smaller  he  would  tind  it.  But  actual 
measurement  would  show  it  to  be  so  near  90°  that  the  dif- 
ference would  be  imperceptible  with  ordinary  instruments. 
Hence,  the  sun  is  really  at  the  point  where  the  dotted  line  and 
the  line  MS  continued  meet  each  other,  which  is  many  times 
the  distance  EM  to  the  moon. 

This  idea  was  applied  by  Aristarchus,  who  flourished  in  the 
third  century  before  Christ,  preceding  both  Ilipparchus  and 
Ptolemy,  to  determine  the  distance  of  the  sun,  or,  more  ex- 
actly, how  many  times  it  exceeded  the  distance  of  the  moon. 
He  found,  by  measurement,  that,  in  the  position  represented 
in  tlie  figure,  the  distance  between  the  directions  of  tlie  sun 
and  moon  was  87°,  and  that  the  sun  was  therefore  something 
like  twenty  times  as  far  as  the  moon.  "We  now  know  that  this 
result  was  twenty  times  too  small,  the  angle  being  really  so 
near  90°  tliat  Aristarchus  could  not  determine  the  difference 
with  certaint3\     In  principle,  the  method  is  quite  correct,  and 


THE  MOON.  23 

very  ingenious,  but  it  cannot  be  applied  in  practice.  The  one 
insuperable  difficulty  of  the  method  arises  from  the  impossi- 
bility of  seeing  when  the  moon  is  exactly  half  illuminated, 
the  uncertainty  arising  from  tlie  inequalities  in  the  lunar  sur- 
face being  greater  than  the  whole  angle  to  be  measured. 

AVatching  and  mapping  down  the  path  of  the  moon  among 
the  stars,  it  is  found  not  to  be  the  same  with  that  of  the  sun, 
being  inclined  to  it  about  5°.  The  paths  cross  each  other  in 
two  opposite  points  of  the  heavens,  called  the  moon's  nodes. 
The  path  of  the  moon  in  the  middle  of  the  year  1877  is 
marked  on  star  Maps  1I.~V.  Referring  to  Map  III.,  it  will 
be  seen  that  the  descending  node  of  the  moon  is  in  the  con- 
stellation Leo,  very  near  the  star  Regulus.  Here  the  moon 
passes  south  of  or  below  the  ecliptic,  and  continues  below  it 
over  the  whole  of  Map  IV.  On  Map  V.,  it  approaches  the 
ecliptic  again,  crossing  to  the  north  of  it  in  the  constellation 
Aquarius,  and  continuing  on  that  side  till  it  reaches  Regulus 
once  more. 

Such  is  the  moon's  path  in  July,  1877.  But  it  is  con- 
stantly changing  in  consequence  of  a  motion  of  the  nodes 
towards  the  west,  amounting  to  more  than  a  degree  in  ever' 
revolution.  In  order  that  the  line  drawn  on  the  map  may 
continue  to  represent  the  path  of  the  moon,  we  must  suppose 
it  to  slide  along  the  ecliptic  towards  the  right  at  the  rate  of 
about  20°  a  year,  so  that  a  slightly  different  path  will  be  de- 
scribed in  every  monthly  revolutioji.  The  path  will  always 
cross  the  ecliptic  at  the  same  angle,  but  the  moon  will  not 
always  pass  over  the  same  stars.  In  August,  1877,  she  will 
cross  the  ecliptic  a  little  farther  to  the  rigiit  (west),  and  will 
pass  a  little  below  Regr.lus.  The  change  going  on  from 
month  to  month  and  from  year  to  year,  in  a  little  less  than 
ten  year&  the  ascending  node  will  be  found  in  Leo ;  and  the 
other  node,  now  in  Leo,  will  liave  gone  back  to  Aquarius. 
In  a  period  of  eighteen  years  and  seven  months,  the  nodes 
will  have  made  a  complete  revolution,  and  the  path  of  the 
moon  will  have  resumed  the  position  given  on  the  map. 


24       SYSTEM  OF  THE   WORLD  HISTORICALLY  DEVELOrED. 

§  6.  Eclipses  of  the  Su?i  and  Moon. 

The  early  inhabitants  of  the  world  were,  uo  doubt,  terrified 
by  the  occasional  recurrence  of  eclipses  many  ages  before 
there  were  astronomers  to  explain  their  causes.  But  the  mo- 
tions of  the  sun  and  moon  could  not  be  observed  very  long 
without  the  causes  being  seen.  It  was  evident  that  if  the 
moon  should  ever  chance  to  pass  between  the  earth  and  the 
sun,  she  must  cut  off  some  or  all  of  his  light.  If  the  two  bodies 
followed  the  same  track  in  the  heavens,  there  would  be  an 
eclipse  of  the  sun  every  new  moon ;  but,  owing  to  the  incli- 
nation of  the  two  orbits,  the  moon  will  generally  pass  above 
or  below  tlie  sun,  and  there  will  be  no  eclipse.  If,  however, 
the  sun  happens  to  be  in  the  neighborliood  of  the  moon's  node 
when  the  moon  passes,  then  there  will  be  an  eclipse.  For  an 
example,  let  us  refer  to  Maj)  III.  AV^e  see  that  the  sun  passes 
the  moon's  descending  node  about  August  25th,  1877,  and  is 
within  20"  of  this  node  from  early  in  August  till  the  middle 
of  September.  The  moon  passes  the  sun  on  August  Sth  and 
September  6tli  of  that  year,  which  are,  therefore,  the  dates  of 
new  moon.  At  the  first  date,  the  moon  passes  so  far  to  tlie 
north  that,  as  seen  from  the  centre  of  the  earth,  there  is  no 
eclipse  at  all;  but  in  tlie  northern  part  of  Asia  the  moon 
would  be  seen  to  cut  off  a  small  portion  of  the  sun. 

While  the  moon  is  performing  another  circuit,  the  sun  has 
moved  so  far  past  the  node,  that  the  moon  passes  south  of  it, 
and  there  is  only  a  small  eclipse,  and  that  is  visible  only 
around  the  rogion  of  Cape  Horn.  Thus,  there  are  two  solar 
eclipses  whll*^  the  sun  is  passing  this  node  in  1877,  but  both 
are  very  small.  Inderd,  every  time  the  sun  crosses  a  node, 
the  moon  is  sure  to  cross  his  path,  eitlier  before  he  reaches 
the  node,  or  before  lie  gets  far  enough  from  it  to  be  out  of 
the  way.  As  he  crosses  both  nodes  in  the  course  of  the  year, 
there  must  be  at  least  two  solar  eclipses  every  year  to  some 
points  of  the  earth's  surface. 

The  cause  of  lunar  eclipses  might  not  have  been  so  easy  to 
guess  as  was  that  of  solar  ones ;  but  a  great  number  could 


ECLIPSES  OF  THE  SUN  AND  MOON.  25 

not  liave  been  observed,  and  their  times  of  occurrence  record- 
ed, without  its  being  noticed  that  they  always  occurred  at  full 
moon,  when  the  earth  was  opposite  the  sun.  The  idea  that 
the  earth  cast  a  shadow,  and  that  the  moon  passed  into  it, 
could  then  hardly  fail  to  suggest  itself ;  and  we  find,  accord- 
ingly, that  the  earliest  observers  of  the  heavens  were  perfectly 
acquainted  with  the  cause  of  lunar  eclipses. 

The  reason  why  eclipses  of  the  moon  only  occur  occasion- 
ally is  of  the  same  general  nature  with  that  of  the  rare  occur- 
rence of  solar  eclipses.  The  centre  of  the  earth's  shadow  is 
always,  like  the  sun,  in  the  ecliptic ;  and  unless  the  moon  hap- 
pens to  be  very  near  the  ecliptic,  and  therefore  very  near  one 
of  her  nodes  at  the  time  of  full  moon,  she  will  fail  to  strike 
the  shadow,  passing  above  or  below  it.  Owing  to  the  great 
magnitude  of  the  sun,  the  earth's  sliadow  is,  at  the  distance  of 
the  moon,  much  smaller  than  the  earth  itself.  The  result  of 
this  is,  that  the  moon  must  be  decidedly  nearer  her  node  to 
produce  a  lunar  than  to  produce  a  solar  eclipse.  Sometimes 
a  whole  year  passes  without  there  being  any  eclipse  of  the 
moon. 

The  nature  of  an  ec^lipse  will  vary  with  the  positions  and 
apparent  magnitudes  of  the  sun  and  moon.  Let  us  suppose, 
first,  that,  in  a  solar  eclipse,  the  centre  of  the  moon  happens 
to  pass  exactly  over  the  centre  of  the  sun.  Then,  it  is  clear 
that  if  the  apparent  angular  diameter  of  the  moon  exceed  that 
of  the  sun,  the  latter  will  be  entirely  hidden  from  view.  This 
is  called  a  total  eelij.)se  of  the  su7i.  It  is  evident  that  such  ai. 
ecllDfeo  can  occur  only  when  the  observer  is  near  the  line  join- 
ing the  centres  of  the  sufi  and  moon.  If,  under  the  same  cir- 
cumstances, the  apparent  magnitude  of  the  moon  is  less  than 
that  of  the  sun,  it  is  evident  that  the  whole  of  the  latter  cannot 
be  covered,  but  a  ring  of  light  around  his  edge  will  still  be  visi- 
ble. Tliis  is  called  an  annular  eclipse.  If  the  moon  does  not 
pass  centrally  over  the  sun,  then  it  can  cover  only  a  portion  of 
the  latter  on  one  side  or  the  other,  and  the  eclipse  is  said  to  be 
partial.  So  with  the  moon :  if  the  latter  is  only  partially  im- 
mersed in  the  earth's  shadow,  the  eclipse  of  the  moon  is  called 


26       SYSTEM  OF  THE  WORLD  HISTORICALLY  DEVELOPED. 

partial;  if  slie  is  totally  immersed  in  it,  so  that  no  direct  sun- 
light can  reach  her,  the  eclipse  is  said  to  be  total.     An  an- 


Fio.  6. — Auuular  eclipse  of  the  sun. 


Fig.  v.— Paitial  eclipee  of  the  sun. 


nular  eclipse  of  the  moon  is  impossible,  because  the  earth's 
shadow  always  exceeds  the  diameter  of  the  moon  in  breadth. 

Some  points  respecting  eclipses  will  be  seen  more  clearly 
by  reference  to  the  accompanying  figures,  in  which  S  repre- 
sents the  sun,  E  the  earth,  and  M  the  moon.  Referring  to  the 
first  figure,  it  will  be  seen  that  an  observer  at  either  of  the 
points  marked  O,  or  indeed  anywhere  outside  the  shaded  por- 
tions, will  see  the  whole  of  the  sun,  so  that  to  him  there  will 
be  no  eclipse  at  all.  Within  the  liglitly  shaded  regions,  marked 
PP,  the  sun  will  be  partially  eclipsed,  and  more  so  as  the  ob- 
server is  near  the  centre.    This  region  is  called  the  penumbra. 


Fio.  8 — Eclipse  of  the  suu,  the  shadow  of  the  moon  falling  on  the  earth. 

Within  the  darkest  parts  between  the  two  letters  iP  is  a  region 
where  ♦^ho  sun  is  totally  hidden  by  the  moon.  This  is  the 
shadow,  and  its  form  is  that  of  a  cone,  with  its  base  on  the 
moon,  and  its  point  extending  towards  the  earth.  Now,  it 
happens  that  the  diameters  of  the  sun  and  moon  are  very 
nearly  proportional  to  their  respective  mean  distances,  so  that 
the  point  of  this  shadow  almost  exactly  reaches  the  surface  of 
the  earth.  Indeed,  so  near  is  the  adjustment,  that  the  dark 
shadow  sometimes  reaches  the  earth,  and  sometimes  does  not, 


ECLIPSES  OF  THE  SUN  AND  MOON. 


owing  to  the  small  changes  in  the  distance  of  the  snn  and 
moon.  When  the  shadow  reaches  the  earth,  it  is  comparative- 
ly very  narrow,  owing  to  its  being  so  near  its  sharp  point ;  but 
if  an  observer  can  station  himself  within  it,  he  will  see  a  total 
eclipse  of  the  sun  during  the  short  time  the  shadow  is  passing 
over  him.  If  the  reader  will  study  the  figure,  he  will  see  why 
a  total  eclipse  of  the  sun  is  so  rare  at  any  one  place  on  the 
earth.  The  shadow,  when  it  reaches  the  earth,  is  so  near  down 
to  a  point  that  its  diameter  is  not  i^enerally  more  than  a  hun- 
dred miles ;  consequently,  each  total  eclipse  is  visible  only 
along  a  belt  which  may  not  average  more  than  a  handred 
miles  across. 

In  most  eclipses,  the  shadow  comes  to  a  point  before  it 
reaches  the  earth ;  in  this  case,  the  apparent  angular  diameter 
of  the  moon  is  less  than  that  of  the  sun,  and  there  can  be  no 
total  eclipse.  But  if  an  observer  places  himself  in  a  line  with 
the  centre  of  the  shadow,  he  will  see  an  annular  eclipse,  the 
sun  showing  itself  on  all  sides  of  the  moon. 

The  next  fio-ure  shows  us  the  form  of  the  earth's  shadow. 


Fio.  9.— Eclipse  of  the  moon,  the  latter  being  in  the  shadow  of  the  earth. 

The  earth  being  much  larger  than  the  moon,  its  shadow  ex- 
tends far  beyond  it ;  and  where  it  reaches  the  moon,  it  is  al- 
ways so  much  larger  than  the  latter  that  she  may  be  wholly 
immersed  in  it,  as  shown  in  the  figure.  Kow,  suppose  the 
moon,  in  her  course  round  the  earth,  to  pass  centrally  through 
the  sliadow,  and  not  above  or  below  it,  as  she  commonly  does ; 
then,  when  she  entered  the  shaded  region,  marked  P,  which 
is  called  the  penumbra,  an  observer  on  her  surface  would  see 
a  partial  eclipse  of  the  sun  caused  by  the  intervention  of  the 


28       SYSTEM  OF  THE  WORLD  HISTOBICALLY  DEVELOPED. 

earth.  Tlie  time  when  this  begins  is  given  in  the  almanacs, 
being  expressed  by  the  words,  "  Moon  enters  penumbra." 
Some  of  the  sunlight  is  then  cut  off  from  the  moon,  so  that 
the  latter  is  not  so  bright  as  usual ;  but  the  eye  does  not 
notice  any  loss  of  light  until  the  moon  almost  reaches  the 
dark  shadow.  As  she  enters  the  shadow,  a  portion  of  her  sur- 
face seems  to  be  cut  off  and  to  disappear  entirely,  and  her  vis- 
ible portion  continually  grows  smaller,  until,  in  case  of  a  total 
eclipse,  her  whole  disk  is  innnersed  in  the  shadow.  When  this 
occurs,  it  is  found  that  she  is  not  entirely  invisible,  but  still 
faintly  shines  with  a  lurid  copper-colored  light.  This  light  is 
refracted  into  the  shadow  by  the  earth's  atmosphere,  and  its 
amount  may  be  greater  or  less,  according  to  the  quantity  of 
clouds  and  vapor  in  the  atmosphere  around  that  belt  of  the 
earth  which  the  sunlight  must  graze  in  order  to  reach  the  moon. 

In  about  half  of  the  lunar  eclipses,  the  moon  passes  so  far 
above  or  below  the  centre  of  the  shadow  that  part  of  her  body 
is  in  it,  and  part  outside,  at  the  time  of  greatest  eclipse.  This 
is  called  ii  partial  eclipse  of  the  moon.  The  magnitude  of  a 
partial  eclipse,  whether  of  the  sun  or  moon,  was  measured  by 
the  older  astronomers  in  digits.  The  diameter  of  the  solar  or 
lunar  disk  was  divided  into  twelve  equal  parts,  called  digits; 
and  the  magnitude  of  the  eclipse  was  said  to  be  equal  to  the 
number  of  die-its  cut  off  bv  the  shadow  of  the  earth  in  case  of 
a  lunar  eclipse,  or  by  the  moon  in  case  of  a  solar  eclipse.  The 
most  ancient  astronomers  were  in  the  liabit  of  measuring  the 
digits  by  surface :  when  the  moon  was  said  to  be  eclipsed  four 
digits,  it  meant  that  one -third  of  her  surface,  and  not  one- 
third  her  diameter,  was  eclipsed. 

The  duration  of  an  eclipse  varies  between  very  wide  limits, 
according  to  whether  it  is  nearly  central  or  the  contrary.  The 
duration  of  a  solar  eclipse  depends  upon  the  time  required  for 
the  moon  to  pass  over  the  distance  from  where  she  first  comes 
into  apparent  contact  with  the  sun's  disk,  until  she  separates 
from  it  again  ;  and  this,  in  the  case  of  eclipses  which  are  pret- 
ty large,  may  range  between  two  and  three  hours.  In  a  total 
eclipse,  however,  the  apparent  disk  of  the  moon  exceeds  that 


ECLIPSES  OF  THE  SUN  AND  MOON.  29 

of  the  sun  by  so  small  an  amount,  that  it  takes  her  but  a  short 
time  to  pass  far  enough  to  uncover  some  part  of  the  sun's 
disk;  the  time  is  rarely  more  than  live  or  six  minutes,  and 
sometimes  only  a  few  seconds.  A  total  eclipse  of  the  moon 
may,  however,  last  nearly  two  hours,  and  the  partial  eclipses 
on  each  side  of  the  total  one  may  extend  the  whole  duration 
of  the  eclipse  to  three  or  four  hours. 

Total  eclipses  of  the  sun  afford  very  rare  and  highly  prized 
opportunities  for  studying  the  operations  going  on  around  that 
luminary.     Of  these  we  shall  speak  in  a  subsequent  chapter. 

Returning,  now,  to  the  apparent  motions  of  the  sun  and 
moon  around  the  celestial  sphere,  we  see  that  since  the  moon's 
orbit  has  two  opposite  node's  in  which  it  crosses  the  ecliptic, 
and  the  sun  passes  through  the  entire  course  of  the  ecliptic  in 
the  course  of  the  year,  it  follows  that  there  are  two  periods  in 
the  course  of  a  year  during  which  the  sun  is  near  a  node,  and 
eclipses  may  occur.  Roughly  speaking,  these  periods  are  each 
about  a  month  in  duration,  and  we  may  call  them  seasons  of 
eclipses.  For  instance,  it  will  be  seen  on  Map  V.  that  the 
sun  passes  one  node  of  the  moon's  orbit  towards  the  end  of 
February,  1877.  A  season  of  eclipses  for  that  year  is  there- 
fore February  and  the  first  half  of  March.  Actually,  there  is 
a  total  eclipse  of  the  moon  on  February  27th,  and  a  very  small 
eclipse  of  the  sun  on  March  14th,  of  that  year,  visible  only  in 
Northern  Asia.*  From  this  time,  the  sun  is  so  far  from  the 
node  that  there  can  be  no  eclips<  s  until  he  approaches  the 
other  node  in  August.  Then  we  have  the  two  eclipses  of  the 
sun  already  mentioned,  and,  between  them,  a  total  eclipse  of 
the  moon  on  August  23d.  Thus,  in  the  year  1877,  the  first 
season  of  eclipses  is  in  February  and  March,  and  the  second 
in  August  and  September. 

We  have  said  that  the  length  of  eacli  eclipse  season  is  about 
a  month.  To  speak  with  greater  accuracy,  the  average  season 
for  eclipses  of  the  sun  extends  18  days  before  and  after  the 

*  There  is  an  extraordinary  coincidence  between  this  eclipse  and  that  of  Au- 
gust 8th  of  the  same  year,  both  being  visible  from  nearly  tlie  same  region  in  Cen- 
tral Siberia. 


30       SYSTEM  OF  THE  WORLD  HISTORICALLY  DEVELOPED. 

sun's  passage  tlirongh  the  jiode,  wliile  tliat  for  lunar  eclipses 
extends  11^  days  on  each  side  of  the  node.  Tlie  total  season 
is,  therefore,  36  days  for  solar,  and  23  days  for  lunar  eclipses. 

Owing  to  the  constant  motion  of  the  moon's  node  already 
described,  the  season  of  eclipses  will  not  be  the  same  from 
year  to  year,  but  will  occur,  on  tlie  average,  about  20  days 
earlier  each  year.  We  have  seen  that  the  sun  passed  the  de- 
scending node  of  the  moon  marked  on  Map  III.  on  August 
24th,  1877;  but  during  the  year  following  tlie  node  will  have 
moved  so  far  to  the  west  that  the  sun  will  again  reach  it  on 
August  5tli,  1878.  The  effect  of  this  constant  shifting  of  the 
nodes  and  seasons  of  eclipses  is  that  in  1887  the  August  sea- 
son will  be  shifted  back  to  February,  and  the  February  season 
to  August.  The  reader  who  wishes  to  find  the  middle  of  the 
eclipse  seasons  for  twenty  or  thirty  years  can  do  so  by  starting 
from  March  1st  and  August  24:th,  1877,  and  subtracting  19f 
days  for  each  subsequent  year. 

There  is  a  relation  between  the  motions  of  the  sun  and 
moon  which  materially  assisted  the  early  astronomers  in  the 
prediction  of  eclipses.  AVe  have  said  that  the  moon  makes 
one  revolution  among  the  stars  in  about  27^  days.  Since  the 
node  of  the  orbit  is  constantly  moving  back  to  meet  the  moon, 
as  it  were,  she  will  return  to  her  node  in  a  little  less  than  this 
period — namely,  as  shown  by  modern  observations,  in  a  mean 
interval  of  27.21222  days.  The  sun,  after  passing  any  node 
of  the  orbit,  will  reach  the  sam.e  node  again  in  346.6201  days. 
The  relation  between  these  numbers  is  this  :  242  returns  of 
the  moon  to  a  node  take  very  nearly  the  same  time  with  19 
returns  of  the  sun,  the  intervals  being 

242  returns  of  the  moon  to  hei  node B/jBa.Sf)?  days; 

19       "         "      sun  to  moon's  node 6585.780     " 

Consequently,  if  at  any  time  the  sun  and  moon  should  start 
ont  together  from  a  node,  they  would,  at  the  end  of  6585 
days,  or  18  years  and  11  days,  be  again  found  together  very 
near  the  same  node.  During  the  interval,  there  would  have 
been  223  new  and  full  moons,  but  none  so  near  the  node  as 


ECLIPSES  OF  THE  SUN  AND  MOON.  31 

this.  The  exact  time  required  for  223  lunations  is  G5 85.32 12 
days;  so  that,  in  the  case  supposed,  the  223d  conjunction  of 
the  sun  and  moon  would  happen  a  little  before  tliey  reached 
the  node,  their  distance  from  it  being,  by  calculation,  a  little 
less  than  one  of  their  diameters,  or,  more  exactly,  28'.  If, 
instead  of  being  exactly  at  tlie  node,  tliey  are  any  given  dis- 
tance from  it,  say  3°  east  or  west,  then,  in  the  same  period, 
they  will  be  again  together  within  half  a  degree  of  the  same 
distance  from  the  node. 

The  period  just  found  was  called  the  Saros,  and  may  be  ap- 
plied in  tins  way:  Let  us  note  the  exact  time  of  the  middle 
of  any  eclipse,  either  of  the  moon  or  of  the  sun ;  then  let  us 
count  forwards  G585  days,  7  hours,  42  minutes,  and  we  sliall 
find  another  eclipse  of  very  nearly  the  same  kind.  Reduced 
to  years,  tlie  interval  will  be  18  years  and  10  or  11  days,  ac- 
cording to  whctlier  the  29th  of  February  has  intervened  four 
or  five  times  during  the  interval.  This  being  true  of  every 
eclipse,  if  we  record  all  the  eclipses  which  occur  during  a 
period  of  18  years,  we  shall  find  the  same  series  after  10  or 
11  days  to  begin  over  again ;  but  the  new  series  will  not  gen- 
erally be  visible  at  the  same  places  with  the  old  ones,  or,  at 
least,  will  not  occur  at  the  same  time  of  day,  since  the  mid- 
dle will  be  nearly  eight  hovirs  later.  Not  till  the  end  of  three 
periods  will  they  recur  near  the  same  meridian ;  and  then, 
owing  to  the  period  not  being  exact,  the  eclipse  will  not  be 
precisely  of  the  same  magnitude,  and,  indeed,  may  fail  entire- 
ly. Every  successive  recurrence  of  an  eclipse  at  the  end  of 
the  period  being  28'  farther  back  relatively  to  the  node,  the 
conjunction  must,  in  process  of  time,  be  so  far  back  fi'om  the 
node  as  not  to  produce  an  eclipse  at  all.  During  nearW  every 
period  it  will  be  found  that  some  eclipse  fails,  and  that  some 
new  one  enters  in.  A  new  eclipse  of  the  moon  thus  entering 
will  be  a  very  small  one  indeed.  At  every  successive  recur- 
rence of  its  period  it  will  be,  larger,  until,  about  its  thirteenth 
recurrence,  it  will  be  total.  It  will  be  total  for  about  twenty- 
two  or  twenty-three  recurrences,  when  it  will  become  partial 
once  more,  but  on  the  opposite  side  of  the  moon  from  that  on 


32       SYSTEM  OF  TTIE  WOULD  HISTORICALLY  DEVELOPED. 

vvliicli  it  was  first  seen.  There  will  then  be  about  thirteen  par- 
tial eclipses,  each  smaller  than  the  last,  until  they  fail  entirely. 
The  whole  interval  of  time  over  which  the  recurrence  of  a 
lunar  eclipse  thus  extends  will  be  about  48  periods,  or  865^ 
years.  The  solar  eclipses,  occurring  farther  from  the  node, 
will  last  yet  longer,  namely,  from  65  to  70  periods,  or  over 
1200  years. 

As  a  recent  example  of  the  Saros,  we  may  cite  some  total 
eclipses  of  the  sun  well  known  in  recent  times ;  for  instance, 

1842,  July  8th,  V"  8  a.m.,  total  eclipse,  observed  in  Europe; 

18(j(),  July  18th,  9''  a.m.,  total  eclipse  America  and  Spain  ; 

1878,  July  29th,  •^'^  2  p.m.,  one  visible  in  Colorado  and  on  the  Pacific  Coast. 

A  yet  more  remarkable  series  of  total  eclipses  of  the  sun 
occui-8  in  the  years  1850, 1868, 1886,  etc.,  the  dates  being — 

1850,  August  7th,  4''  4  p.m.,  in  the  Pacific  Ocean ; 

18()8,  August  17th,  12''  p.m.,  in  India; 

1886,  August  2!)th,  S*"  a.m.,  in  the  Central  Atlantic  Ocean  and  Southern  Africa; 

1904,  September  9th,  noon,  in  South  America. 

This  series  is  remarkable  for  the  long  duration  of  totality, 
amounting  to  some  six  minutes. 

It  must  be  understood  that  the  various  numbers  we  have 
given  in  this  section  are  not  accurate  for  all  cases,  because  the 
motions  both  of  the  sun  and  moon  are  subject  to  certain  small 
irregularities  which  may  alter  the  times  of  eclipses  by  an  hour 
or  more.  We  have  given  only  mean  values,  which  are,  how- 
ever, always  quite  near  the  truth. 

§  7.   The  Ptolemaic  Syde7n. 

There  is  still  extant  a  work  which  for  fourteen  centuries 
was  a  sort  of  astronomical  Bible,  from  which  nothing  was 
taken,  and  to  which  nothing  material  in  principle  was  added. 
This  is  the  "Almagest"  of  Ptolemy,  composed  about  the  mid- 
dle of  the  second  century  of  our  era.  Nearly  all  we  know  of 
the  ancient  astronomy  as  a  science  is  derived  from  it.  Frag- 
ments of  other  ancient  authors  have  come  down  to  us,  and 
most  of  the  ancient  writers  make  occasional  allusions  to  astro- 
nomical phenomena  or  theories,  from  which  various  ideas  re- 


THE  PTOLEMAIC  SYSTEM.  33 

specting  the  ancient  astronomy  liave  been  gleaned;  but  the 
work  of  Ptolemy  is  the  only  complete  compendium  which  we 
possess.  Although  his  system  is  in  several  important  points 
erroneous,  it  yet  represents  the  salient  features  of  the  apparent 
motions  of  the  heavenly  bodies  with  entire  accuracy.  Defec- 
tive as  it  is  when  measured  by  our  standard,  it  is  a  marvel  of 
ingenuity  and  research  when  measured  by  the  standard  of  the 
times. 

The  immediate  object  of  the  present  chapter  is  to  explain 
the  apparent  movements  of  the  planets,  which  can  be  most 
easily  done  on  the  Ptolemaic  system.  But,  on  account  of  its 
historic  interest,  we  shall  begin  with  a  brief  sketch  of  the 
propositions  on  which  the  system  rests,  giving  also  Ptolemy's 
method  of  proving  them.  His  fundamental  doctrines  are  that 
the  heavens  are  spherical  in  form,  and  all  the  heavenly  mo- 
tions spherical  or  in  circles ;  that  the  earth  is  also  spherical, 
and  situated  in  the  centre  of  the  heavens,  or  celestial  sphere, 
where  it  remains  quiescent,  and  that  it  is  in  magnitude  only  a 
point  when  compared  with  the  sphere  of  the  stars.  We  shall 
give  Ptolemy's  views  of  these  propositions,  and  his  attempts 
to  prove  them,  in  their  regular  order. 

1st.  The  Heavenly  Bodies  move  in  Circles. — Here  Ptole- 
my refers  principally  to  the  diurnal  motion,  whereby  every 
heavenly  body  is  apparently  carried  around  tlie  earth,  or,  rath- 
er, around  the  pole  of  the  heavens,  in  a  circle  every  day.  But 
all  the  ancient  and  mediaeval  astronomers  down  to  the  time 
of  Kepler  had  a  notion  that,  the  circle  being  the  most  perfect 
plane  figure,  all  tlie  celestial  motions  must  take  place  in  cir- 
cles; and  as  it  was  found  that  the  motions  were  never  uni- 
form, they  supposed  these  circles  not  to  be  centred  on  the 
earth.  Where  a  sino;le  circle  did  not  suffice  to  account  for 
the  motion,  they  introduced  a  combination  of  circular  motions 
in  a  manner  to  be  described  presently. 

2d.  The  Earth  is  a  Sj)he7'e.  —  That  the  earth  is  rounded 
from  east  to  west  Ptolemy  proves  by  the  fact  that  the  sun, 
moon,  and  stars  do  not  rise  and  set  at  the  same  moment  to  all 
the  inhabitants  of  the  earth.     The  times  at  which  eclipses  of 

4 


34       SYSTEM  OF  TUE  WOULD  HISTORICALLY  DEVELOPED. 

the  moon  are  seen  in  different  countries  being  compared,  it  is 
found  that  the  farther  tlie  observer  is  west,  the  earlier  is  the 
hour  after  sunset.  As  tlie  time  is  really  the 'same  everywhere, 
this  shows  that  the  sun  sets  later  the  farther  we  go  to  the  west. 
Again,  if  the  earth  were  not  rounded  from  north  to  south,  a 
star  passing  the  meridian  in  the  north  or  south  horizon  would 
always  pass  in  the  horizon,  however  far  to  the  north  or  south 
the  observer  might  travel.  But  it  is  found  that  when  an  ob- 
server travels  towaruo  the  south,  the  stars  in  the  north  ap- 
])roach  ^he  horizon,  and  the  circles  of  their  diurnal  motion  cut 
below  it,  while  nr  '  sir>-rs  rise  into  view  above  the  south  hori- 
zon. This  shov'o  i>  i'  the  horizon  itself  changes  its  direction 
as  the  observer  raovco.  Finallv,  from  whatever  direction  we 
approach  elevated  objt';ts  from  the  sea,  we  see  that  their  bases 
arc  fi'  t  hidden  from  view  by  the  curvature  of  the  watei,  and 
gradual.      "e  into  view  ar;  we  approach  them. 

3d.  The  Earil  '  in  the  Centre  of  the  Celestial  Sphere. — 
If  the  earth  wcr,.  ...  ^»'i'  ed  from  the  centre,  there  would  be 
various  a.  """HHties  in  t'le  a})parent  daily  motion  of  the  ce- 
lestial spV  ji'e,  riic  siais  appearing  to  move  faster  on  the  side 
towards  wliich  the  earth  was  situated.  If  it  were  displaced 
towards  the  east,  we  should  be  nearer  the  heavenly  bodies 
when  they  arc  rising  tlnn  when  they  are  setting,  and  they 
would  appear  to  move  more  rapidly  in  the  east  than  in  the 
west.  The  forenoons  would  therefore  be  shorter  than  the  af- 
ternoons. Towards  whate\^.  side  of  the  turning  sphere  it 
might  be  moved,  the  heavenly  bodies  would  seem  to  move 
more  rapidly  on  thnt  side  than  on  the  other.  No  such  irreg- 
ularity being  seen,  but  the  diurnal  motion  taking  place  with 
perfect  uniformity,  the  earth  must  be  in  the  centre  of  mo- 
tion. 

4th.  The  Earth  has  no  Motion  of  Translation  —  Because 
if  it  had  it  would  move  aw^a^,  from  the  centre  towards  one 
side  of  the  celestial  sphere,  ana  the  diurnal  revolution  of  the 
stars  would  cease  to  be  uniform  in  all  its  parts.  But  the  uni- 
formity of  motion  just  described  being  seen  from  year  to  year, 
the  earth  must  preserve  its  position  in  the  centre  of  the  sphere. 


THE  PTOLEMAIC  SYSTEM.  P>5 

It  will  be  iiitcrostiiig  to  analyze  these  propositions  of  Ptole- 
my, to  see  what  is  true  and  what  is  false.  The  first  proposi- 
tion—  that  the  heavenly  bodies  move  in  circles,  or,  as  it  is 
more  literally  exj)ressed,  that  the  heavens  move  spherically — 
is  (piite  true,  so  far  as  the  a[)parent  diurnal  motion  is  con- 
cerned. What  Ptolemy  did  not  know  was  that  this  motion  is 
only  apparent,  arising  from  a  rotation  of  the  earth  itself  on  its 
axis.  The  second  proposition  is  perfectly  correct,  and  Ptole- 
my's proofs  that  the  earth  is  round  are  liiose  still  found  i"  our 
school-books  at  the  end  of  seventeen  hundred  years,  xuost 
curious,  however,  is  the  mixture  of  truth  and  falsehood  in  the 
third  and  fourth  propositions,  that  the  earth  remains  (piies- 
cent.  We  cannot  denounce  it  as  unqualifiedly  false,  because, 
in  a  certain  sense,  and  indeed  in  the  only  sense  in  which  there 
is  any  celestial  sphere,  the  earth  may  be  said  to  remain  in  the 
centre  of  the  sphere.  What  Ptolemy  did  not  see  is  that  this 
sphere  is  only  an  ideal  one,  which  the  spectator  carries  with 
him  wherever  he  goes.  Ilis  demonstration  that  the  centre  of 
revolution  of  the  sphere  is  in  the  earth  is,  in  a  certain  sense, 
correct ;  but  what  he  really  ])roves  is  that  the  earth  revolves 
on  its  own  axis.  lie  did  not  see  that  if  the  earth  conld  carry 
the  axis  of  revolution  with  it,  his  demonstration  of  the  quies- 
cence of  the  earth  would  fall  to  the  «;round. 

Considerable  insight  into  Ptolemy's  views  is  gained  by  his 
answers  to  two  objections  against  his  system.  The  first  is  the 
vulgar  and  natural  one,  that  it  is  paradoxical  to  suppose  that 
a  body  likv.^  the  earth  could  remain  supported  on  nothing,  and 
still  be  at  rest.  These  objectors,  he  says,  reason  from  what 
they  see  happen  to  small  bodies  around  them,  and  not  from 
what  is  proper  to  the  universe  at  large.  There  is  neither  up 
nor  down  in  the  celestial  spaces,  for  we  cannot  conceive  of  it 
in  a  sphere.  What  we  call  down  is  simply  the  direction  of 
our  feet  towards  the  centre  of  the  earth,  the  direction  in 
which  heavy  bodies  tend  to  fall.  The  earth  itself  is  but  a 
point  in  comparison  with  the  celestial  spaces,  and  is  kept  fixed 
by  the  forces  exerted  upon  it  on  all  sides  by  the  universe, 
which  is  infinitely  larger  than  it,  and  similar  in  all  its  parts. 


36       SYSTEM  OF  THE  WOULD  HISTORICALLY  DEVELOPED. 

This  idea  is  as  near  an  approach  to  that  of  universal  gravita- 
tion as  the  science  of  the  times  would  admit  of. 

lie  tlien  says  there  are  others  who,  admitting  this  reason- 
ing, pretend  that  notliing  hinders  ns  from  snj)posing  that  the 
heavens  are  immovable,  and  that  the  earth  itself  turns  round 
its  own  axis  once  a  day  from  west  to  east.  It  is  certainly 
singular  that  one  who  liad  risen  so  far  above  the  inusi(ms  of 
sense  as  to  demonstrate  to  the  world  that  the  earth  was  round ; 
that  up  and  down  were  only  relative ;  and  that  heavy  bodies 
fell  towards  a  centre,  and  not  in  some  unchangeable  direction, 
should  not  have  seen  the  correctness  of  this  view. 

To  refute  the  doctrine  of  the  earth's  rotation,  he  proceeds 
in  a  way  the  opposite  of  that  which  he  took  to  refute  those 
who  thought  the  earth  could  not  rest  on  nothing.  lie  said  of 
the  latter  that  they  regarded  solely  what  was  around  them  on 
the  earth,  and  did  not  consider  what  was  proper  to  the  uni- 
verse at  large.  To  those  who  maintained  the  earth's  rotation, 
he  says,  if  we  consider  only  the  movements  of  the  stars,  there 
is  nothing  to  oppose  their  doctrine,  which  he  admits  has  the 
merit  of  simplicity ;  but  in  view  of  what  passes  around  us  and 
in  the  air,  their  doctrine  is  ridiculous.  He  then  entei's  into  a 
disquisition  on  tlie  relative  motion  of  light  and  heavy  bodies, 
which  is  extremely  obscure ;  but  his  conclusion  is  that  if  the 
earth  really  rotated  with  the  enormous  velocity  necessary  to 
carry  it  round  in  a  day,  the  air  would  be  left  behind.  If  they 
say  that  the  earth  carries  round  the  air  with  it,  he  replies  that 
this  could  not  be  true  of  bodies  floating  in  the  air;  and  hence 
concludes  that  the  doctrine  of  the  earth's  rotation  is  not  tena- 
ble. It  is  clear,  from  this  argument,  that  if  Ptolemy  and  his 
contemporaries  had  devoted  to  experimental  pliysics  half  the 
careful  observation,  research,  and  reasoning  which  we  find  in 
their  astronomical  studies,  they  could  not  have  failed  to  estab- 
lish the  doctrine  of  the  earth's  rotation. 

In  the  Ptolemaic  system,  all  the  celestial  motions  are  repre- 
sented by  a  series  of  circular  motions.  We  have  already  ex- 
plained the  motions  of  the  sun  and  moon  among  the  stars,  the 
first  describing  a  complete  circuit  of  the  heavens  from  west  to 


THE  PTOLEMAIC  SYSTEM.  87 

east  in  a  year,  and  the  second  a  similar  circuit  in  a  month. 
Though  not  entirely  uniform,  these  movements  are  always  for- 
ward. But  it  is  not  so  with  the  five  planets  —  Mercury,  Ve- 
nus, Mars,  Jupiter,  and  Saturn.  These  move  sometimes  to  the 
east  and  sometimes  to  the  west,  and  are  sometimes  stationary.''*" 
On  the  whole,  however,  the  easterly  movements  predominate ; 
and  the  planets  really  oscillate  around  a  certain  mean  point 
itself  in  regular  motion  towards  the  east.  Let  us  take,  for  in- 
stance, the  planet  Jupiter.  Suppose  a  certain  fictitious  Jupi- 
ter perfoi-ming  a  circuit  of  the  heavens  among  the  stai's  every 
twelve  years  with  a  rejjular  easterly  motion,  iust  as  the  sun 
performs  such  a  circuit  every  year;  then  the  real  Jupiter  will 
be  found  to  oscillate,  like  a  pendulum,  on  each  side  of  the  fic- 
titious planet,  but  never  swinging  more  than.  12°  from  it.  The 
time  of  each  double  oscillation  is  about  thirteen  months — that 
is,  if  on  January  1st  we  find  it  passing  the  fictitious  planet 
towards  the  west,  it  will  continue  its  westerly  swing  about 
three  months,  when  it  will  gradually  stop,  and  return  with  a 
somewhat  slower  motion  to  the  fictitious  planet  again,  passing 
to  the  east  of  it  the  middle  of  Jul}'.  The  easterly  swing  will 
continue  till  about  the  end  of  October,  when  it  will  return 
towards  the  west.  The  westerly  or  backward  motion  is  called 
retrograde^  and  the  easterly  motion  direct.  Between  the  two 
is  a  point  at  which  the  planet  appears  stationary  once  more. 
The  westerly  motions  are  called  retrograde  because  they  are 
in  the  opposite  direction  both  to  the  motion  of  the  sun  among 
the  stars,  and  to  the  average  direction  in  which  all  the  planets 
move.  It  was  seen  by  llipparchus,  who  lived  three  centuries 
before  Ptolemy,  that  this  oscillating  motion  could  be  repre- 
sented by  supposing  the  real  Jupiter  to  describe  a  circular  or- 
bit around  the  fictitious  Jupiter  once  in  a  year.  This  orbit  is 
called  the  epicycle,  and  thus  we  have  the  celebrated  epicyclic 
theory  of  the  planetary  motions  laid  down  in  the  "  Almagest." 
The  movement  of  the  planet  on  this  theory  can  be  seen  by 

*  It  miiy  not  be  amiss  to  remind  the  reader  once  more  that  we  here  leave  the 
diurnal  motion  of  the  stars  entirely  out  of  sight,  and  consider  only  the  motions  of 
the  planets  relative  to  the  stars. 


38       SYSTEM  OF  TEE  WORLD  HISTORICALLY  DEVELOPED. 

Fig.  10.  E  is  the  earth,  around  which  the  fictitious  Jupiter 
moves  in  tlie  clotted  circle,  1, 2,  3, 4,  etc.  To  form  the  epicycle 
in  which  the  real  plaijet  moves,  we  must  suppose  an  arm  to  be 
constantly  turning  round  the  fictitious  planet  once  a  year,  on 
the  end  of  which  Jupiter  ib.  carried.  This  arm  will  then  be  in 
the  successive  positions,  11/,  2  2',  3  3',  etc.,  represented  by  the 
light  dotted  lines.  Drawing  a  line  through  the  successive  po- 
sitions 1',  2',  3',  etc.,  of  the  real  Jupiter,  we  shall  have  a  series 
of  loops  representing  its  apparent  orbit. 


Fio.  10.— Showing  the  apparent  orbit  of  a  planet,  regarding  the  earth  as  at  rest. 

It  will  be  seen  that  although  it  requires  only  a  year  for  the 
arm  carrying  the  real  Jupiter  to  perform  a  complete  revolu- 
tion and  return  to  its  primitive  direction,  it  requires  about 
thirteen  months  to  form  a  complete  loop,  because,  owing  to 
the  motion  of  the  fictitious  planet  in  its  orbit,  the  arm  must 
move  more  than  a  complete  j'evolution  to  finish  the  loop.  For 
instance,  referring  again  to  Fig.  10,  comparing  the  positions 
11'  and  8  8',  it  will  be  seen  that  the  arm,  being  in  the  same 
direction,  has  performed  a  complete  revolution;  but,  owing  to 
the  curvature  of  the  orbit,  it  does  not  reach  the  middle  of  the 
second  loop  until  it  attains  the  position  9  9'. 


THE  PTOLEMAIC  SYSTEM. 


39 


The  planets  of  which  the  radius  of  the  epicycle  makes  an 
annual  revolution  in  this  way  are  Mars,  Jupiter,  and  Saturn. 
The  complete  apparent  orbits  of  the  last  two  planets  are  shown 
in  the  next  figure,  taken  from  Arago.  By  the  radius  of  the 
epicycle  we  mean  the  imaginary  revolving  arm  which,  turn- 
ing round  the  fictitious  planet,  carries  the  real  planet  at  its 


20 3 w 

Fig.  11.— Apparent  orbitd  of  Jupiter  and  Saturu,  1708-1 T37,  after  Cassini. 

end.  The  law  of  revolution  of  this  arm  is,  that  whenever  the 
planet  is  opposite  the  sun,  the  arm  points  towards  the  earth, 
as  in  the  positions  1 1',  9  9',  in  which  cases  the  sun  will  Le  on 
the  side  of  the  earth  opposite  the  planet ;  while,  whenever  the 
planet  is  in  conjunction  with  the  sun,  the  arm  points  from  the 
earth.  This  fact  was  well  known  to  the  ancient  astronomers, 
and  their  calculations  of  the  motions  of  the  planets  were  all 


40       SYSTEM  OF  THE  WOULD  HISTORICALLY  DEVELOPED. 

founded  upon  it;  but  they  do  not  seem  to  have  noticed  the 
very  important  corollary  from  it,  that  the  direction  of  the 
radius  of  the  epicycle  of  Mars,  Jupiter,  and  Saturn  is  always 
the  same  with  that  of  the  sun  from  the  earth.  Had  they 
done  so,  they  could  hardly  have  failed  to  see  that  the  epicycles 
could  be  abolished  entirely  by  supposing  that  it  was  the  earth 
which  moved  round  the  sun,  and  not  the  sun  round  the  earth. 
The  peculiarity  of  the  planets  Mercury  and  Yen  us  is  that 
the  fictitious  centres  around  which  they  oscillate  are  always  in 
the  direction  of  the  sun,  or,  as  we  now  know,  the  sun  himself 
is  the  centre  of  their  motions.  They  are  never  seen  more  than 
a  limited  distance  from  that  luminary,  Venus  oscillating  about 
45°  on  each  side  of  the  sun,  and  Mercury  from  16°  to  29°.  It 
is  said  that  the  ancient  Egyptians  really  did  make  the  sun  the 
centre  of  the  motion  of  these  two  planets ;  and  it  is  difficult  to 
see  how  any  one  could  have  failed  to  do  so  after  learning  the 
laws  of  their  oscillation.  Yet  Ptolemy  rejected  this  system, 
placing  their  orbits  between  the  earth  and  sun  without  assign- 
ing any  good  reason  for  the  course. 

The  arrangement  of  the  planets  on  the  Ptolemaic  system  is 
shown  in  Fig.  12.  The  nearest  planet  is  the  moon,  of  which 
the  ancient  astronomers  actually  succeeded  in  roughly  meas- 
uring the  distance.  The  remaining  planets  are  arranged  in 
the  same  order  with  their  real  distance  from  the  sun,  except 
that  the  latter  takes  the  place  assigned  to  the  earth  in  tlie 
modern  system.     Thus  we  have  the  following  order : 

The  Moon, 

Mercury, 

Yenus, 

The  Sun, 

Mars, 

Jupiter, 

Saturn. 
Outside  of  Saturn  was  the  sphere  of  the  fixed  stars. 

This  order  of  the  planets  must  have  been  a  matter  of  opin- 
ion rather  than  of  demonstration,  it  being  correctly  judged 
by  the  ancient  astronomers  that  those  which  seemed  to  move 


THE  PTOLEMAIC  SYSTEM. 

Saticrn 


41 


Pio.  12.— Arrnngement  of  the  seven  planets  iu  the  Ptolemaic  system.  The  orbits,  as 
marked,  are  those  of  t'^e  flctitioul  planets/  the  real  planets  beinf;  supposed  to  describe 
a  series  of  loops. 

more  slowly  were  the  more  distant.  This  system  made  it 
quite  certain  that  the  moon  was  the  nearest  planet,  and  Mars, 
Jupiter,  and  Saturn,  in  their  order,  the  most  distant  ones.  But 
the  relative  positions  of  the  Sun,  Mercury,  and  Venus  were 
more  in  doubt,  since  they  all  performed  a  revolution  round 
the  celestial  sphere  in  a  year.  So,  while  Ptolemy,  as  we  have 
just  said,  placed  Mercury  and  Venus  between  the  earth  and 
the  sun,  Plato  placed  them  beyond  the  sun,  tlie  order  being. 
Moon,  Sun,  Mercury,  Venus,  Mars,  Jupiter,  Saturn. 

Hipparchus  and  Ptolemy  made  a  series  of  investigations  re- 
specting the  times  of  revolution  of  the  planets,  and  the  inequal- 
ities of  their  motions.      which  it  is  worth  while  to  give  a  brief 


42       SYSTEM  OF  THE  WOULD  UISTORICALLY  DEVELOPED. 


summary.  The  former  was  no  doubt  an  abler  astronomer  than 
Ptolemy ;  but  as  he  was,  so  far  as  we  know,  the  first  accurate 
observer  of  the  celestial  motlous,  he  could  not  make  a  suf- 
ficiently long  series  of  observations  to  determine  all  the  peri- 
ods of  tlie  planets.  Ptolemy  had  the  advantage  of  being  able 
to  combine  his  own  observations  with  those  of  Ilipparchus, 
three  centuries  earlier. 

Imperfect  though  their  means  of  observation  were,  these 
observers  found  that  the  easterly  movements  of  the  planets 
amonir  the  stars  were  none  of  them  uniform.  This  held  true 
not  only  of  the  sun  and  moon,  but  of  the  fictitious  planets 

already  described.  Hence  they 
invented  the  eccentric,  and  sup- 
posed the  motions  to  be  really  cir- 
cular and  uniform,  but  in  circles 
not  centred  in  the  earth.  In  Fig. 
18,  let  E  be  the  earth,  and  C  the 
centre  around  which  the  planet 
really  revolves.  Then,  when  the 
planet  is  passing  the  point  /^, 
which  is  nearest  the  earth,  its  an- 
gular motion  would  seem  more 
rapid  than  the  average,  because 
in»  general  the  angular  velocity 
of  a  moving  body  is  greater  the 
nearer  the  observer  is  to  it,  while 
when  passing  A  it  will  seem  to  be 
more  slow  than  the  average.  The  angular  velocity  being 
always  greatest  in  one  point  of  the  orbit,  and  least  in  a  point 
directly  opposite,  changing  regularly  from  the  maximum  to 
the  minimum,  the  general  features  of  the  movement  are  cor- 
rectly represented  by  the  eccentric.  ]>y  comparing  the  angu- 
lar velocities  in  different  points  of  the  orbit,  Ilipparchus  and 
Ptolemy  were  able  to  determine  the  supposed  distance  of  the 
earth  from  the  centre,  or  rather  the  proportion  of  this  distance 
to  the  distance  of  the  planet.  The  distance  thus  determined 
is  double  its  true  amount.     The  point  P  is  called  the  Perigee, 


Pig.  13. —  The  eccentric.  Shows  how 
^the  iiiiciei.it-'  rei)reseiite(l!the  unequii! 
apparent  velocitie.H  of  Uie  planets 
when  their  real  motion  was  8n|)pose(l 
uniform,  by  placing  the  earth  away 
from  the  centre  of  motlou,  at  E. 


THE  PTOLEMAIC  SYSTEM.  43 

and  A  the  Apogee.  The  distance  C£^  from  the  earth  to  the 
centre  of  motion  is  the  eccentricity.  As  there  was  no  way  of 
deterniinin<r  the  absohite  dimensions  of  the  orbit,  it  was  neces- 
sary  to  take  the  ratio  of  6'^'  to  the  radius  of  the  orbit  6'jP  or 
CEiov  the  eccentricity.* 

In  determining  the  motions  of  the  moon,  Ilipparchus  and 
Ptolemy  depended  ahnost  entirely  on  observations  of  lunar 
eclipses.  The  first  of  these,  it  is  said,  was  observed  at  Babylon 
in  the  lirst  year  of  Mardocempad,  between  the  29th  and  30th 
days  of  the  Egyptian  month  Thoth.  It  commenced  a  little 
more  than  an  hour  after  the  moon  rose,  and  was  total.  The 
date,  in  our  reckoning,  was  b.c.  720,  March  19th.  The  series 
of  eclipses  extended  from  this  date  to  that  of  Ptolemy  him- 
self, who  lived  between  eight  and  nine  centuries  later.  If  the 
observations  of  these  eclipses  had  been  a  little  more  precise, 
they  would  still  be  of  great  value  to  us  in  fixing  the  mean 
motion  of  the  moon.  As  it  is,  we  can  now  calculate  the  cir- 
cumstances of  an  ancient  eclipse  from  our  modern  tables  of 
the  sun  and  moon  almost  as  accurately  as  any  of  the  ancient 
astronomers  could  observe  it. 

Notwithstanding  the  extremely  imperfect  character  of  the 
observations,  both  Ilipparchus  and  Ptolemy  made  discoveries 
respecting  the  peculiarities  of  the  moon's  motions  which  shew 
'  a  most  surprising  depth  of  research.  By  comparing  tlie  inter- 
vals between  eclipses,  they  found  that  her  motion  was  not  uni- 
form, but  that,  like  the  sun,  she  moved  faster  in  some  parts  of 
her  orbit  tlian  in  others.  To  account  fur  this,  they  supposed 
her  orbit  eccentric,  like  that  of  the  sun  ;  that  is,  the  earth,  in- 
stead of  being  in  the  centre  of  the  circular  orbit  of  the  moon, 
was  supposed  to  be  displaced  by  about  a  tenth  part  the  whole 
distance  of  that  bodj'.  So  far  the  orbit  of  the  moon  was  like 
that  of  the  sun  and  the  fictitious  planets,  except  that  its  eccen- 
tricity was  greater.     But  a  long  series  of  observations  showed 

*  Compared  with  the  modern  theory  of  the  elliptic  motion,  approximately  treat- 
ed, the  distance  CE  is  double  the  eccentricity  of  the  ellipse.  One-half  the  appar- 
ent inequality  is  really  caused  by  the  orbit  being  at  various  distances  from  the 
earth  or  sun,  but  tlie  other  half  is  real. 


44       SYSTEM  OF  THE  WORLD  HISTORICALLY  DEVELOPED. 

that  the  perigee  and  apogee  did  not,  as  in  the  case  of  the  sun 
and  phmets,  remain  in  the  same  points  of  tiie  orbit,  but  moved 
forwards  at  sucli  a  rate  as  to  carry  tliem  round  the  heavens  in 
nine  years ;  tliat  is,  supposing  Fig.  13  to  represent  the  orbit  of 
the  moon,  the  centre  of  the  circle  C  revolved  round  the  earth 
in  nine  years,  and  the  orbit  changed  its  position  accordingly. 

It  was  also  found  by  Ptolemy,  by  measuring  the  apparent 
angle  between  the  moon  and  sun  in  various  points  of  the 
orbit  of  the  former,  that  there  was  yet  another  inequality  in 
her  motion.  This  has  received  the  name  of  the  evection.  In 
consequence  of  this  inequality,  the  moon  oscillates  more  than 
a  degree  on  each  side  of  her  position  as  calculated  from  the 
eccentric,  in  a  period  not  differing  much  from  lit  r  revolution 
round  the  earth.  To  represent  this  motion,  Ptolemy  had  to 
introduce  a  small  additional  epicycle,  as  in  the  case  of  the 
planets,  only  the  radius  was  so  small  that  there  was  no  looping 
of  the  orbit.  In  consequence,  his  theory  of  the  moon's  motion 
was  quite  complicated;  yet  he  managed  to  represent  this  mo- 
tion, within  the  limits  of  the  errors  of  his  observations,  by  a 
combination  of  circular  motions,  and  thus  saved  the  favorite 
theory  of  tlie  times,  that  all  the  celestial  motions  were  circular 
and  uniform. 

§  8.  The  Calendar. 

One  of  the  earliest  purposes  of  the  study  of  the  celestial 
motions  was  that  of  finding  a  convenient  measurement  of 
time.  This  application  of  astronomy,  being  of  great  antiquity, 
having  been  transmitted  to  us  without  any  fundamental  altera- 
tion, and  depending  on  the  apparent  motions  of  the  sun  and 
moon,  which  we  have  studied  in  this  chapter,  is  naturally  con- 
sidered in  connection  with  the  ancient  astronomy. 

The  astronomical  divisions  of  time  are  the  day,  the  month, 
and  the  year.  The  week  is  not  such  a  division,  because  it  does 
not  correspond  to  any  astronomical  cycle,  altliough,  as  we  shall 
presently  see,  a  certain  astronomical  signification  was  said  to 
have  been  given  to  it  by  the  ancient  astrologers.  Of  these 
divisions  the  day  is  the  most  well-marked  and  striking  through- 


THE  CALENDAR.  45 

out  the  habitable  portion  of  the  globe.  Had  a  people  lived  at 
or  near  the  polef^  it  would  have  been  less  striking  than  the  year. 
But  wherever  '  an  existed,  tliere  was  a  regular  alternation  of 
day  and  night,  with  a  corresponding  alternation  in  his  physical 
condition,  both  occurring  with  such  regularity  and  uniformity 
as  to  furnish  in  all  asjes  the  most  definite  unit  of  time.  For 
merely  chronological  purposes  the  day  would  have  been  the 
only  unit  of  time  theoretically  necessary ;  for  if  mankind  had 
begun  at  some  early  age  to  number  every  day  by  counting 
from  1  forwards  without  limit,  and  had  every  historical  event 
been  recorded  in  connection  with  the  number  of  the  day  on 
which  it  happened,  there  would  have  been  far  less  uncertain- 
ty about  dates  than  now  exists.  But  keeping  count  of  such 
large  numbers  as  would  have  accunnilated  in  the  lapse  of  cen- 
turies would  have  been  very  inconvenient,  and  a  simple  count 
of  time  by  days  has  never  been  used  for  the  purposes  of  civil 
life  through  any  greater  period  than  a  single  month. 

Next  to  the  day,  the  most  definite  and  striking  division  of 
time  is  the  year.  The  natui-al  year  is  that  measured  by  the 
retui-n  of  tlie  seasons.  All  the  operations  of  agriculture  are 
80  intimately  dependent  on  this  recurrence,  that  man  must 
have  beffun  to  make  use  of  it  for  nieasurino;  time  lone;  before 
he  had  fully  studied  the  astronomical  cause  on  which  it  de- 
pends. The  years  in  the  lifetime  of  any  one  generation  not 
being  too  numerous  to  be  easily  reckoned,  the  year  was  found 
to  answer  every  purpose  of  measuring  long  intervals  of  time. 

The  number  of  days  in  the  year  is,  however,  too  great  to 
be  conveniently  kept  count  of;  an  intermediate  measure  was 
therefore  necessary.  This  was  suggested  by  the  motion  and 
phases  of  the  moon.  The  "  new  moon  "  being  seen  to  emerge 
from  the  sun's  rays  at  intervals  of  about  30  days,  a  measure 
of  very  convenient  length  was  found,  to  which  a  permanent 
interest  was  attached  by  the  religious  rites  connected  with  the 
reappearance  of  the  moon. 

The  week  is  a  division  of  time  entirely  disconnected  with 
the  month  and  year,  the  employment  of  which  dates  from  the 
Mosaic  dispensation.     The  old  astrologers  divided  the  seven 


4G       SYSTEM  OF  TUB  WOULD  HISTORICALLY  DEVELOPED. 

days  of  the  week  among  the  seven  phmets,  not  in  the  order  of 
tlieir  distance  from  the  sun,  but  in  one  nhown  by  the  foHow- 
ing  figure.  If  we  go  round  the  circle  in  the  direction  of  the 
hands  of  a  watch,  we  shall  find  the  names  of  the  seven  plan- 
ets of  the  ancient  astronomy,  in  the  order  of  their  supposed 
distances  f  while,  if  we  follow  the  lines  drawn  in  the  circle 
from  side  to  side,  we  shall  have  the  days  of  the  week  in  tlieir 
order. 


^^ 


,vUCt  if 


8(i*y 


^clit^y 


Fig.  U.— Showing  the  astrological  division  of  tlie  seven  planets  among  the  days  of  the 

week. 

If  the  lunar  month  had  been  an  exact  number  of  days,  say 
30,  and  the  year  an  exact  number  of  months,  as  12,  there 
would  have  been  no  difficulty  in  the  use  of  these  cycles  for 
the  measurement  of  time.  Rut  the  former  is  several  hours 
less  than  30  days,  w'hile  the  latter  is  nearly  12^  lunar  months. 
In  the  attempt  to  combine  these  measures,  the  ancient  calen- 


*  See  pages  40,  41. 


THE   CALENDAR.  47 

dars  M'ere  thrown  into  a  confusion  which  made  them  very  per- 
plexing, and  which  we  see  to  this  day  in  the  irregular  lengths 
of  our  months.  To  describe  all  the  devices  which  we  know  to 
have  been  nsed  for  remedying  these  difficulties  would  be  very 
tedious ;  we  shall  therefore  confine  ourselves  to  their  general 
nature. 

The  lunar  month,  or  the  mean  interval  between  successive 
new  moons,  is  very  nearly  29^  days.  In  counting  months  by 
the  moon,  it  was  therefore  common  to  make  their  length  29 
and  30  days,  alternately.  IJut  the  period  of  29^  days  is  really 
about  three-quarters  of  an  hour  too  short.  In  the  course  of 
three  years  the  count  will  therefore  be  a  day  in  error,  and  it 
will  be  necessary  to  add  a  day  to  one  of  the  months.  When 
lunar  months  were  used,  the  year,  comprising  12  such  months, 
would  consist  of  only  354  days,  and  would  therefore  be  11 
days  too  short.  Nevertheless,  such  a  year  was  used  both  by 
the  Greeks  and  Homans,  and  is  still  used  by  the  Mahome- 
tans; the  i^omans,  however,  in  the  calendar  of  Numa,  adding 
22  or  23  days  to  every  alternate  year  by  inserting  the  inter- 
calary month  Mercedonius  between  the  23d  and  24th  of  Feb- 
ruary. 

The  irregularity  and  inconvenience  of  reckoning  by  lunar 
months  caused  them  to  be  very  generally  abandoned,  the  only 
reason  for  their  retention  being  relic-ious  observances  due  at 
the  time  of  new  moon,  which,  among  the  Jews  and  other  an- 
cient nations,  were  regarded  as  of  the  highest  importance.  Ac- 
cordingly, we  find  the  Egy])tians  counting  by  months  of  30 
davs  each,  and  makins;  every  vear  consist  of  12  such  months 
and  five  additional  days,  making  3G5  days  in  all.  As  the  true 
length  of  the  year  was  known  to  be  about  six  hours  greater 
than  this,  the  equinox  would  occur  six  hours  later  every  year, 
and  a  month  later  after  the  lapse  of  120  years.  After  the  lapse 
of  1460  years,  according  to  the  calculations  of  the  time,  each 
season  would  have  made  a  complete  course  through  the  twelve 
months,  and  would  then  have  retui-ned  once  more  at  the  same 
time  of  year  as  in  the  beginning.  This  was  termed  the  Sothic 
Period;  but  the  error  of  each  year  being  estimated  a  little 


48        SYSTEM  OF  THE  WORLD  UISWItlCALLY  DEVELOPED. 

too  great,  as  wc  now  know,  the  true  length  of  the  period 
would  have  been  about  1500  years. 

The  confusion  in  the  Greek  year  was  partly  remedied 
through  the  discovery  by  Melon  of  the  cycle  whi'ih  has  since 
borne  his  name.  This  cycle  consists  of  19  solar  years,  during 
which  the  moon  changes  235  times.  Tlie  error  of  this  cycle 
is  very  small,  as  may  be  seen  from  the  following  periods,  com- 
puted from  modern  data : 

Diij-a.       Iloun,     MIn. 

23.")  lunations  leiiuire  in  tlie  mean (lOJiO       IG      31 

IDtrue  soliu-years  (tiopicaij ()!)31)       U       27 

19  Julian  years  of  3G5{  days (;93'J       18        0 

Hence,  if  we  take  235  lunar  months,  and  divide  them  np  as 
nearly  evenly  as  is  convenient  into  19  years,  the  mean  length 
of  these  years  will  be  near  enough  right  for  all  the  purposes 
of  civil  reckoning.  The  years  of  each  cycle  were  numbered 
from  1  to  19,  and  tlie  number  of  the  year  was  called  the  Gold- 
en Number,  from  its  having  been  ordered  to  be  inscribec^  on 
the  monuments  in  letters  of  gold. 

The  Golden  Number  is  still  used  in  our  church  calendars 
for  finding  the  date  of  Easter  Sunday.  This  is  the  solitary 
religious  festival  which,  in  Christian  countries,  depends  on  the 
motion  of  the  moon.  Tlie  nominal  rule  for  determining  East- 
er is  that  it  is  the  Sunday  following  the  first  new  moon  which 
occurs  after  the  21st  of  March.  The  dates  of  the  new  moon 
correspond  to  the  Metonic  (^ycle ;  that  is,  after  the  lapse  of  19 
years  they  recur  on  or  about  tlie  same  day  of  the  year.  Con- 
sequently, if  we  make  a  list  of  the  dates  on  which  the  Paschal 
new  moon  occurs,  we  shall  find  no  two  dates  to  be  the  same 
for  nineteen  successive  years ;  but  the  twentieth  will  occur  on 
the  same  day  with  the  first,  or,  at  most,  only  one  day  different, 
and  then  the  whole  series  will  be  repeated.  Consequently, 
the  Golden  Number  for  the  year  shows,  with  sufficient  exact- 
ness for  ecclesiastical  purposes,  on  what  day,  or  how  many 
days  after  the  equinox,  the  Paschal  new  moon  occurs.  The 
church  calculations  of  Easter  Sunday  are,  however,  founded 
upon  very  old  tab.'es  of  the  moon,  so  that  if  we  fixed  it  by  the 


THE  CALENDAll.  49 

actual  moon,  we  should  often  find  the  calendar  feast  a  week 


in  ciToi: 


The  basis  of  the  calendars  now  employed  throughout  Chris- 
tendom was  laid  by  Julius  Ca3sar.  Previous  to  his  time,  the 
Iloman  calendar  was  in  a  state  of  great  (jonfusion,  the  nomi- 
nal length  of  the  year  depending  very  largely  on  the  caprice 
of  the  ruler  for  the  time  being.  It  was,  however,  very  well 
known  that  the  real  length  of  the  solar  year  was  about  305^ 
days ;  and,  in  order  tliat  the  calendar  year  might  have  the  same 
mean  longtli,  it  was  prescribed  that  the  ordinary  year  siiould 
consist  of  305  days,  but  that  one  day  sliould  be  added  to  every 
fourth  year.  The  lengths  of  the  months,  as  we  iiow  have  them, 
were  finally  arranged  by  the  inmiediate  successors  of  Ca3sar. 

The  Julian  calendar  continued  unaltered  for  about  sixteen 
centuries ;  and  if  the  true  length  of  the  tropical  year  had  been 
3^5^  days,  it  would  have  been  in  use  still.  But,  as  we  have 
seen,  this  period  is  about  11^  minutes  longer  than  the  solar 
year,  a  quantity  which,  repeated  every  year,  amounts  to  an  en- 
tire day  in  128  years.  Consequentl}',  in  the  sixteenth  century, 
the  equinoxes  occurred  11  or  12  days  sooner  than  they  should 
have  occurred  according  to  the  calendar,  or  on  the  10th  in- 
stead of  the  21st  of  March.  To  restore  them  to  their  original 
position  in  the  year,  or,  more  exactly,  to  their  position  at  the 
time  of  the  Council  of  Nice,  was  the  object  of  the  Gregorian 
reformation  of  the  calendar,  so  called  after  Pope  Gregory 
XIII.,  by  whom  it  was  directed.  The  change  consisted  of 
two  parts : 

1.  The  5th  of  October,  158'2,  according  to  the  Julian  calen- 
dar, was  called  the  15th,  the  count  being  thus  advanced  10 
days,  and  the  equinoxes  made  once  more  to  occur  about  March 
21st  and  September  21st. 

2.  The  closing  year  of  each  century,  1600,  1700,  etc.,  in- 
stead of  being  each  a  leap-year,  as  in  the  Julian  calendar, 
should  be  such  only  when  the  number  of  the  century  was  di- 
visible by  4.  While  IGOO,  2000,  2400,  etc.,  were  to  be  leap- 
years,  as  before,  1700,  1800,  1900,  2100,  etc.,  were  to  be  re- 
duced to  3G5  davs  each. 

5 


50       SYSTEM  OF  THE  WORLD  HISTORICALLY  DEVELOPED. 

This  change  in  the  calendar  was  soon  adopted  by  all  Catho- 
lic countries,  and,  more  slowly,  by  Protestant  ones — England, 
among  the  latter,  h.olding  out  for  more  than  a  century,  but 
finally  entering  into  the  change  in  1752.  In  Kussia  it  was 
never  adopted  at  all,  the  Julian  calendar  being  still  continued 
in  that  country.  Conse(piently,  the  Russian  reckoning  is  now 
12  days  behind  ours,  the  10  days'  difference  during  the  six- 
teenth and  seventeentli  centuries  being  increased  by  the  days 
dropped  from  the  years  1700  and  1800  in  the  new  reckoning. 

The  length  of  the  mean  Gregorian  year  is  365*1  5h  49m  12^; 
while  that  of  the  tropical  year,  according  to  the  best  astronom- 
ical determination,  is  3G5''  5'^  48""  46^  The  former  is,  there- 
fore, still  26  seconds  too  long,  an  error  which  will  not  amount 
to  an  entire  day  for  more  than  8000  years.  If  there  were 
any  object  in  having  the  calendar  and  the  astronomical  years 
in  exact  coincidence,  the  Gregorian  year  would  be  accurate 
enough  for  all  practical  purposes  during  many  centuries.  In 
fact,  however,  it  is  difficult  to  show  what  ])ractical  object  is  to 
be  attained  by  seeking  for  any  such  coincidence.  It  is  im- 
portant that  summer  and  winter,  seed-time  and  harvest,  shall 
occur  at  the  same  time  of  the  year  through  several  successive 
generations ;  but  it  is  not  of  the  slightest  importance  that 
they  should  occur  at  the  same  time  now  that  they  did  5000 
years  ago,  nor  would  it  cause  any  difficulty  to  our  descendants 
of  5000  years  hence  if  tlie  equinox  sliould  occur  in  the  middle 
of  February,  as  would  be  the  case  should  the  Julian  calendar 
have  been  continued. 

The  change  of  calendar  met  with  much  popular  opjiosition, 
and  it  may  hereafter  be  conceded  that  in  this  instance  the 
common  sense  of  the  people  was  more  nearly  right  than  the 
wisdom  of  the  learned.  An  additional  complication  was  in- 
troduced into  the  reckoning  of  time  without  any  other  real 
object  tlian  tint  of  making  Easter  come  at  the  right  time. 
As  tiie  end  of  the  century  approaches,  tlie  (piestion  of  making 
1900  a  leap-year,  as  usual,  will  no  doubt  be  discussed,  and  it  is 
possible  that  some  concerted  action  may  be  taken  on  the  part  of 
leading  nations  looking  to  a  return  to  the  old  mode  of  reckoning. 


VOrERNICVS.  51 


CHAPTER  II. 

THE    COrEENICAN    SYSTEM,  OR   THE   TRUE    MOTIONS   OF   THE    HEAV- 
ENLY   BODIES. 

§  1.  Copernicus. 

In  tlio  first  section  of  the  preceding  cliapter  we  described 
the  apparent  diurnal  motion  of  the  lieavens,  whereby  all  the 
heavenly  bodies  appear  to  be  carried  round  in  circles,  thus 
performing  a  revolution  every  day.  Any  observer  of  this  mo- 
tion who  should  suppose  the  earth  to  be  flat,  and  the  direction 
we  call  downward  everywhere  the  same,  would  necessarily  re- 
gard it  as  real.  A  very  little  knowledge  of  geometry  would, 
however,  show  him  that  the  appearance  might  be  accounted 
for  by  supposing  the  earth  to  revolve.  The  seemingly  fatal 
objection  against  this  view  would  be  that,  if  such  were  the 
case,  the  surface  of  the  earth  could  not  remain  level,  and  ev- 
ery thing  would  slide  away  from  its  position.  But  it  was  im- 
possible for  men  to  navigate  the  ocean  without  perceiving  the 
rotundity  of  its  surface,  and  we  have  no  record  of  a  time  when 
it  was  not  known  that  the  earth  was  round.  We  have  seen 
tliat  Ptolemy  not  only  was  acquainted  with  the  true  figure  of 
the  earth,  but  knew  that  in  magnitude  it  was  so  much  smaller 
than  the  celestial  spaces,  or  sphere  of  the  lieavens,  as  to  be  only 
a  point  in  comparison.  lie  had,  therefore,  all  the  knowledge 
necessary  to  enable  him  to  see  that  the  moving  body  was  much 
more  likely  to  be  the  earth  than  to  be  the  sphere  of  the  heav- 
ens. Nevertheless,  he  rejected  the  theory  on  obscure  physical 
grounds,  as  shown  in  the  last  chapter,  the  untenability  of  which 
would  have  been  proved  him  b}^  a  few  very  simple  physical  ex- 
periments. And  although  it  is  known  that  the  doctrine  of  the 
earth's  motion  was  sustained  by  others  in  his  age,  notably  by 
Timocharis,  yet  tlie  weight  of  his  authority  was  so  great  as 


52        SYSTEM  OF  THE  WORLD  HISTORICALLY  DEVELOPED. 

not  only  to  override  all  their  arguments,  but  to  carry  his  views 
through  fourteen  centuries  of  the  intellectual  history  of  man. 

The  history  of  astronomy  during  these  centuries  offers  hard- 
ly anything  of  interest  to  tlie  general  reader.  There  was  no 
telescope  to  explore  the  heavens,  and  no  genius  arose  of  suffi- 
cient force  to  unravel  tlie  maze  of  their  mechanism.  It  was 
mainly  through  the  Arabs  that  any  systematic  knowledge  of 
the  science  was  preserved  for  the  use  of  posterity.  The  as- 
tronomers of  this  people  invented  improved  methods  of  ob- 
serving the  positions  of  the  lieavenly  bodies,  and  were  thus 
able  to  make  improved  tables  of  their  motions.  They  meas- 
ured the  obliquity  of  the  ecliptic,  and  calculated  eclipses  of 
the  sun  and  moon  with  greater  precision  than  the  ancient 
Greeks  could  do.  The  predictions  of  the  science  thus  gradu- 
ally increased  in  accuracy,  but  no  positive  step  was  taken  in 
the  direction  of  discovering  tiie  true  nature  of  the  apparent 
movements  of  the  heavens. 

Tlie  honor  of  first  proving  to  the  world  what  tlie  true  theory 
of  the  celestial  motions  is  belongs  almost  exclusively  to  Coper- 
nicus. It  is  true  that  we  have  some  reason  to  believe  that 
Pythagoras  taught  that  the  sun,  and  not  the  earth,  was  the 
centre  of  motion,  and  that  he  was,  therefore,  the  first  to  solve 
the  great  problem.  But  he  did  not  teach  this  doctrine  public- 
ly, and  the  very  vague  statements  of  his  private  teachings  on 
this  point  which  have  been  handed  downi  to  us  are  so  mixed 
up  with  the  speculations  which  the  Greek  philosophers  com- 
bined with  their  views  of  nature,  that  it  is  hard  to  say  with 
precision  whether  Pythagoras  had  or  had  not  fully  seized  the 
truth.  It  is  certain  that  no  modern  would  receive  the  credit 
of  any  discovery  without  giving  more  convincing  proofs  of  the 
correctness  of  his  views  than  we  have  any  reason  to  sup})ose 
that  Pythagoras  gave  to  his  disciples. 

The  great  merit  of  Copernicus,  and  the  basis  of  his  claim  to 
the  discovery  in  question,  is  that  he  was  not  satisfied  with  a 
mere  statement  of  his  views,  but  devoted  a  large  part  of  the 
labor  of  a  life  to  their  demonst..i,tion,  and  thus  placed  them  in 
such  a  light  as  to  render  their  ultimate  acceptance  inevitable. 


COPERNICUS.  53 

Apart  from  all  questions  of  the  truth  or  falsity  of  his  theory, 
the  great  work  in  which  it  was  developed,  ^^De  lievolutionibiis 
Orhiuta  Coelestiuin''''  would  deservedly  rank  as  the  most  im- 
portant compendium  of  astronomy  which  had  appeared  since 
Ptolemy.  Few  books  have  been  more  completely  the  labor  of 
a  lifetime  than  this.  Copernicus  was  born  at  Thorn,  in  Prus- 
sia, in  1473,  twenty  years  before  the  discovery  of  America, 
but  studied  at  the  University  of  Cracow.  lie  became  an  ec- 
clesiastical dignitary,  holding  the  rank  of  canon  during  a  large 
portion  of  his  life,  and  finding  ample  leisure  in  this  position 
to  pursue  his  favorite  studies.  lie  is  said  to  have  conceived  of 
the  true  system  of  the  world  as  early  as  1507.  He  devoted  the 
years  of  his  middle  life  to  the  observations  and  computations 
necessary  to  the  perfection  of  his  system,  and  comuninicated 
his  views  to  a  few  friends,  but  long  refused  to  publish  them, 
fearing  the  popular  prejudice  which  might  thus  be  excited. 
In  151:0,  a  brief  statement  of  them  was  published  by  his  friend 
Rheticus ;  and,  as  this  was  favorably  received,  he  soon  con- 
sented to  the  publication  of  his  great  work.  The  first  printed 
copy  was  placed  in  his  hands  only  a  few  hours  before  his 
death,  which  occurred  in  May,  1513. 

The  fundamental  principles  of  the  Copernican  system  are 
embodied  in  two  distinct  j)ropositions,  which  have  to  be  proved 
separately,  and  one  of  which  might  have  been  true  without 
the  other  being  so.     They  are  ar  follows : 

1.  The  diurnal  revolution  of  the  heavens  is  only  an  appar- 
ent motion,  caused  by  a  diurnal  revolution  of  the  earth  on  an 
axis  passing  through  its  centre. 

2.  The  earth  is  one  of  the  planets,  all  of  which  revolve 
round  the  sun  as  the  centre  of  motion.  The  true  centre  of 
the  celestial  motions  is  therefore  not  the  earth,  but  the  sun. 
For  this  reason  the  Copernican  system  is  frequently  spoken  of 
in  liistorical  discussions  as  the  "heliocentric  theory." 

The  first  proposition  is  the  one  with  the  proof  of  which  Co- 
pernicus begins.  He  explains  how  an  apparent  motion  may 
result  from  a  real  motion  of  the  person  seeing,  as  well  as  from 
a  motion  of  the  object  seen,  and  thus  shows  that  the  diurnal 


54       SYSTEM  OF  THE  WOULD  EISTOEICALLY  DEVELOFED. 

motion  may  be  accounted  for  just  as  well  by  a  revolution  of 
the  earth  as  by  one  of  the  heavens.  To  sailors  on  a  ship  sail- 
ing on  a  smooth  sea,  the  ship,  and  every  thing  in  it,  seems  to  be 
at  rest  and  the  shore  to  be  in  motion.  Which,  then,  is  more 
likely  to  be  in  motion,  the  earth  or  the  whole  universe  outside 
of  it?  In  whatever  proportion  the  heavens  are  greater  than 
the  earth,  in  the  same  proportion  must  their  motion  be  more 
rapid  to  carry  them  round  in  twenty -four  hours.  Ptolemy 
himself  shows  that  the  heavens  were  so  immense  that  the 
earth  was  but  a  point  in  comparison,  and,  for  any  thing  that 
is  known,  they  may  extend  into  infinity.  Then  we  should  re- 
quire an  infinite  velocity  of  revolution.  Tlierefore,  it  is  far 
more  likely  tliat  it  is  this  comparative  point  that  turns,  and 
that  the  universe  is  fixed,  than  the  reverse. 

The  second  principle  of  the  Copernican  system — that  the 
apparent  annual  motion  of  the  sun  among  the  stars,  described 
in  §  3  of  the  preceding  chapter,  is  really  due  to  an  annual  revo- 
lution of  the  earth  around  the  sun — rests  upon  a  very  beautiful 
result  of  the  laws  of  relative  motion.  This  movement  of  the 
earth  explains  not  only  this  apparent  revolution  of  the  sun, 
but  the  apparent  epicyclic  motion  of  the  planets  described  in 
treating  of  the  Ptolemaic  system. 

In  Fig.  15,  let  S  represent  the  &\\r\^ABCD  the  orbit  of  the 
earth  around  it,  and  the  figures  1,  2,  3,  4,  5, 6,  six  successive 
positions  of  the  earth.  These  positions  would  be  about  two 
weeks  apart.  Also,  let  EFOII  represent  the  apparent  sphere 
of  the  fixed  stars.  Tlien,  an  observer  at  1,  viewing  the  sun  in 
the  direction  l^S',  will  see  him  as  if  he  were  in  the  celestial 
sphere  at  the  point  1',  because,  having  no  conception  of  the 
actual  distance,  the  sun  will  appear  to  him  as  if  actually  among 
the  stars  at  V  which  lie  in  the  same  straight  line  witli  him. 
When  the  earth,  with  the  observer  on  it,  reaches  2,  he  will  see 
the  sun  in  the  direction  26'2',  that  is,  as  if  among  the  stars  in 
2'.  That  is,  during  the  two  weeks'  interval,  the  sun  will  ap- 
parently have  moved  among  the  stars  by  an  angle  equal  to  the 
actual  angular  motion  of  the  earth  around  the  sun.  So,  as  the 
earth  passes  through  the  successive  positions  3, 4,  5,  6,  tlie  sun 


COPERNICUS, 


55 


will  appear  in  the  positions  3',  4',  5',  6',  and  the  motion  of  the 
earth  continuing  all  the  way  round  its  orbit,  the  sun  will  ap- 
pear to  move  through  the  entire  circle  EFGII.  Thus  wx 
have,  as  a  result  of  the  annual  motion  of  the  earth  around  the 
sun,  the  annual  motion  of  f'^e  sun  around  the  celestial  sphere 
already  described  in  the  third  section  of  the  preceding  chapter. 


Fia.  15. 


Let  us  now  see  how  this  same  motion  abolishes  the  compli- 
cated system  of  epicycles  by  which  the  ancient  astronomers 
represented  the  planetary  motions.  A  theorem  on  which  this 
explanation  rests  is  this :  If  an  observer  in  unconscious  mo- 
tion sees  an  ohject  at  rest,  that  object  will  seem  to  him,  to  be 
moving  in  a  direction  opposite  to  his  own,  and  with  an  equal 
velocity.    A  familiar  instance  of  this  is  the  apparent  motion 


56        SYSTIJM  OF  THE   WOULD  HISTOIilCALLY  DEVELOPED. 


C' 


of  objects  on  shore  to  passengers  on  a  steamer.  In  Fig.  16, 
let  us  suppose  an  observer  on  the  earth  carried  around  the 

sun  S  in  the  orbit  ABCBEF, 
but  imafj-ining  himself  at  rest 
in  tlie  centre  of  motion  /iS'.  Sup- 
pose that  lie  observes  tho  ap- 
parent motion  of  the  plaiiet  7^, 
which  is  really  at  rest.  IIow 
will  the  planet  appear  to  move  'i 
To  show  this,  we  represent  ap- 
parent directions  and  motions 
by  dotted  lines.  Let  us  begin 
with  the  observer  at  A,  from 
which  position  he  really  sees 
the  planet  in  tho  direction  and 
distance  AP.  Ihit,  imagining 
himself  at  S,  he  thinks  he  sees 
the  planet  at  the  point  «,  the 
distance  and  rection  of  which 
Sa  is  the  same  with  AP.  As 
\f  he  passes  unconsciously  from  A 
to  B,  the  planet  seems  to  him  to 
move  past  from  a  to  i  in  the  op- 
posite direction ;  and, still  think- 
ing himself  at  rest  in  S.,  he  sees 
the  planet  in  Z»,  the  line  Sh  be- 
Fio.  lo.-showinr;  i,ow  tho  nppavont  opi-  \^^„  ^qual  aud  parallel  to  BP. 

cyclic  niDlioii  i)f  till' iilaiK'ts  is  acrountiHl     .       ,  i  p 

for  by  Uie  motion  of  tho  earth  rouiiil  the    As    lie    rCCcdcS    fl'Om    tllO    plail- 

'""•  et  through  the  arc  BCD,  the 

planet  seems  to  recede  from  him  through  bed.  While  he 
moves  from  left  to  right  through  DE.,  the  planet  seems  to 
move  from  right  to  left  through  de.  Finally,  as  he  a]>proaclies 
the  planet  through  the  arc  EFA.,  the  planet  will  seem  to  ap- 
proach him  through  cfa,  and  when  he  gets  back  to  A  he 
will  locate  the  planet  at  a,  as  in  the  beginning.  Thus,  in 
consequence  of  the  motion  of  the  observer  around  the  circle 
ABCBEF,  the  planet,  though  really  at  rest,  will  seem  to  him 


COPERNICUS.  57 

to  move  throngli  a  corresponding  circle,  ahcdef.  If  there  are 
a  number  of  planets,  they  will  all  seem  to  describe  correspond- 
ing circles  of  the  same  magnitude. 

If  the  planet  I\  instead  of  being  at  rest,  is  in  motion,  the 
apparent  circular  motion  will  be  combined  with  the  forward 
motion  of  the  planet,  and  the  latter  will  now  describe  a  circle 
around  a  centre  which  is  in  motion.  Thus  we  have  the  appar- 
ent motion  of  the  planets  around  a  moving  centre,  as  already 
described  in  the  Ptolemaic  system.  We  have  said,  in  §  7  of 
the  preceding  chapter,  that  by  this  system  the  motions  of  the 
planets  are  represented  by  supposing  a  fictitious  planet  to  re- 
volve around  the  heavens  with  a  regular  motion,  while  the 
real  planet  revolves  around  this  fictitious  one  as  a  centre  once 
a  year.  Here,  the  2^i"og7'essive  motion  of  the  fidit'ous  2^l<^i^net 
is  {in  the  case  of  the  outer  j)l<-i'^fGts  3lars,  Ju])iter.,  and  Sat- 
urn) the  motion  of  the  real  j^lo^net  around  the  sun,  while  the 
circle  idhich  the  real  jplanet  describes  around  this  moving  cen- 
time is  only  an  a2ij)are?it  motion  dice  to  the  observer  being  car- 
ried around  the  sun  07i  the  earth.  If  the  reader  will  com- 
pare the  epicyclic  motion  of  Ptolemy,  represented  in  Pigs.  10 
and  11  with  the  motion  explained  in  Pig.  IG,  he  will  find  that 
they  correspond  in  every  particular.  In  the  case  of  the  inner 
planets.  Mercury  and  Venus,  which  never  recede  far  fi-oni  the 
sun,  the  epicyclic  motion  by  which  they  seem  to  vibrate  from 
one  side  of  the  sun  to  the  other  is  due  to  their  orbital  motion 
around  the  sun,  while  the  progressive  motion  with  which  they 
follow^  the  sun  is  due  to  the  revolution  of  the  earth  around 
the  sun. 

AVe  may  now  see  clearly  how  the  retrograde  motion  and 
stationary  phases  of  the  planets  are  explained  on  the  Coper- 
nican  system.  The  earth  and  all  the  planets  are  really  mov- 
ing round  the  sun  in  a  direction  which  we  call  east  on  the 
celestial  sphere.  When  the  earth  and  an  outer  planet  are 
on  the  same  side  of  the  sun,  thev  are  moving  in  the  same 
direction ;  but  the  earth  is  moving  faster  than  the  planet. 
Hence,  to  an  observer  on  the  earth,  the  planet  seems  to  be 
moving  west,  though  its  real  motion  is  east.     As  the  earth 


58       SYSTEM  OF  THE  WORLD  HISTORICALLY  DEVELOPED. 

])asscs  to  the  opposite  side  of  the  sun  from  the  planet,  it 
clianges  its  motion  to  a  direction  the  opposite  of  that  of  the 
planet,  and  thus  the  westerly  motion  of  the  latter  appears  to 
be  increased  by  the  whole  motion  of  tlie  earth.*  Between 
these  two  motions  there  is  a  point  at  which  tlie  planet  does 
not  seem  to  move  at  all  TJiis  is  called  the  stationary  point. 
If  the  planet  we  consider  is  not  an  outer,  but  an  inner  one. 
Mercury  or  Venus,  and  we  view  it  when  between  us  and  tlie 
sun,  its  motion  to  us  is  reversed,  because  we  see  it  from  the 
side  opposite  the  sun.  Hence  it  seems  to  move  west  to  us, 
and  it  is  retrograde.  The  earth  is  indeed  moving  in  the  same 
real  direction ;  but  since  the  planet  moves  faster  tlian  the 
earth,  its  retrograde  motion  seems  to  predominate.  As  the 
planet  passes  round  in  its  orbit,  it  first  appears  stationary, 
and  then,  passing  to  the  opposite  side  of  the  sun,  it  seems 
direct. 

Let  us  now  dwell  for  a  moment  on  some  considerations 
which  will  enable  us  to  do  justice  to  the  Ptolemaic  system,  as 
it  is  called,  by  seeing  how  necessary  a  step  it  was  in  tlie  evo- 
lution of  tlie  true  theory  of  the  universe.  The  great  merit  of 
that  system  consisted  in  the  analysis  of  the  seemingly  compli- 
cated motions  of  the  planets  into  a  combination  of  two  circular 
motions,  the  one  that  of  a  fictitious  planet  around  the  celestial 
sphere,  the  other  that  of  the  real  planet  around  Lio  fictitious 
one.  AVithrut  that  separation,  the  constant  oscillations  of  the 
planets  back  and  forth  could  not  have  suggested  any  idea 
whatever,  except  that  of  a  motion  too  complicated  to  be  ex- 
plained on  mechanical  principles.  But  when,  leaving  out  of 
sii>:ht  the  reii:ular  forward  motion  of  the  mean  or  fictitious 
planet,  the  attention  was  directed  to  the  epicyclic  motion 
alone,  one  could  not  fail  to  see  the  remarkable  correspondence 
between  this  latter  motion  and  the  ap[)arent  annual  motion 
of  the  sun.     Seeing  this,  it  took  a  very  small  step  to  see  that 


*  It  must  not  be  forgotten  that  the  direction  east  in  the  heavens  is  a  curved  di- 
rection, as  it  were,  and  is  opposite  on  opposite  sides  of  tiie  sun  or  celestial  spliere. 
For  instance,  the  motions  of  the  stars  as  they  rise  and  as  they  set  are  o])posite, 
but  botii  are  considered  west. 


COPERNICUS.  59 

tlie  sun,  and  not  the  earth,  was  the  centre  of  planetary  motion. 
Then  nothing  but  the  ilhisions  of  sense  remained  to  prevent 
the  acceptance  of  the  theory  that  the  earth  was  itself  a  planet 
movinn;  rou.nd  the  sun,  and  that  both  the  annual  motion  of  the 
sun  and  the  epicyclic  motion  of  the  planets  were  not  real,  but 
apparent  motions,  due  to  the  motion  of  the  earth  itself ;  and 
in  no  other  way  than  this  could  the  heliocentric  theory  have 
been  developed. 

The  Copernican  system  affords  the  means  of  determining 
tlie  proportions  of  the  solar  system,  or  the  relative  distances  of 
the  several  planets,  with  great  accuracy.  That  is,  if  we  take 
as  our  measuring -rod  the  distance  of  the  earth  from  the  sun, 
we  can  determine  how  many  lengths  of  this  rod,  or  what  frac- 
tional parts  of  its  length,  will  give  the  distance  of  each  planet, 
although  the  length  of  the  rod  itself  may  remain  unknown. 
This  determination  rests  on  the  principle  that  the  apparent 
circle  or  epicycle  described  by  the  planet  in  Fig.  16  is  of  the 
same  magnitude  with  the  actual  orbit  described  by  the  earth 
around  the  sun.  Hence,  the  nearer  the  observer  is  to  this  cir- 
cle, the  larger  it  will  appear.  The  apparent  epicycle  described 
by  Neptune  is  rather  less  than  two  degrees  in  radius;  that  is, 
the  true  planet  Neptune  is  seen  to  swing  a  little  less  than  two 
degrees  on  each  side  of  its  mean  position  in  consequence  of 
the  annual  motion  of  the  earth  rouna  the  sun.  This  shows 
that  the  orbit  of  the  earth,  as  seen  from  Neptune,  subtends  an 
angle  of  only  two  degrees.  On  the  other  hand,  the  planet 
Mars  generally  swings  more  than  40°  on  each  side ;  sometimes, 
indeed,  more  than  45°.  From  this  a  trigonometrical  calcula- 
tion shows  that  its  mean  distance  is  only  about  half  as  much 
again  as  that  of  the  earth ;  and  the  fact  that  the  apparent 
swing  is  variable  shows  the  distance  to  be  different  at  different 
times. 

As  It  will  be  of  interest  to  see  how  nearly  Copernicus  was 
able  to  determine  the  distances  of  the  planets,  we  present  his 
results  in  the  following  table,  together  with  what  we  no\v 
know  to  be  the  true  numbers.  The  numbers  given  are  deci- 
mal fractions,  expressing  the  least  and  greatest  distance  of 


60       SYSTEM  OF  THE  WORLD  BISTOIilCALLY  DEVELOPED. 

each  planet  from  the  sun,  the  distance  of  the  earth  being  taken 
as  unity.* 


riauets. 

Least  Distance. 

GltKATKST 

D[8TAN0E. 

C'opcrnicii?, 

Modern.      ! 

Copernicus. 

Modern. 

Mercury 

Venus 

o.;<2r) 

0.7()lt 

r).4,-);5 

9.70 

o.;?o8     i 
0.718     : 
i.;]y2     j 
r).4.'>4 

10.07 

0.40.^> 
0. 7;50 
1. (!(;(! 
4.980 
8.GG 

0.4G7 
0.728 
l.GGG 
4.9r,2 
9.00 

Mars 

tJiipiter 

ISaturn 

Considering  the  extremely  imperfect  means  of  observation 
which  the  times  afforded,  these  results  of  Copernicus  come 
very  near  the  truth.  The  greatest  proportional  deviation  is  in 
the  case  of  Mercury,  the  most  difficult  of  all  the  planets  to 
observe,  even  to  the  present  day.  It  is  said  that  Copernicus 
died  without  ever  seeing  this  planet. 

The  eccentricities  of  the  orbits  were  represented  by  Coper- 
nicus in  a  way  which  agrees  exactly  with  the  modern  formulae 
when  only  a  rough  approximation  is  sought  for.  Like  Ptole- 
my, he  supposed  the  orbits  of  the  planets  not  to  be  centred  on 
the  sun,  but  to  be  dis})laced  by  a  small  quantity  termed  the 
eccentricity.  But  it  had  long  been  known  that  the  theory  of 
uniform  motion  in  an  eccentric  circle,  though  it  might  make 
the  irregularities  in  the  planet's  angular  motion  come  out  all 
rio;ht,  would  make  the  chano-es  of  distance  double  their  true 
value.  He  therefore  took  for  the  eccentricity  a  mean  between 
that  which  would  satisfy  the  motion  in  longitude,  and  that 
which  would  give  the  changes  of  distance,  and  added  a  small 
epicycle  of  one-third  this  eccentricity ;  and,  by  supposing  the 
planet  to  make  two  revolutions  in  this  epicycle  for  every 
revolution  around  the  sun,  he  represented  both  irregulari- 
ties, f 

*  I  have  deduced  these  numbers  from  the  tables  given  in  Book  V.  of  "De 
Revohitionibus  Orbium  C'oelestium."  They  are  probably  the  most  accurate  that 
Coperni  us  was  able  to  obtain. 

t  Tlie  matiiematical  form  of  this  theory  of  Copernicus  is  as  follows  :  Putting 


OliLIQUITY  OF  THE  ECLIPTIC.  Gl 

The  work  of  Copernicus  Wcas  the  greatest  step  ever  taken  in 
astronomy.  J3ut  he  still  took  little  more  than  the  single  step 
of  showing  what  apparent  motions  in  the  heavens  were  real, 
and  wliat  were  due  to  the  motion  of  the  observer.  Not  only 
was  his  work  in  other  respects  founded  on  that  of  Ptolemy, 
but  he  had  many  of  the  notions  of  the  ancient  philosophy  re- 
specting the  fitness  of  things.  Like  Ptolemy,  he  thought  the 
heavens  as  well  as  the  earth  to  be  spherical,  and  all  the  celes- 
tial motions  to  be  circular,  or  com])osed  of  circles.  He  argues 
against  Ptolemy's  objections  to  the  theory  of  the  earth's  mo- 
tion, that  that  philosopher  treats  of  it  as  if  it  were  an  enforced 
or  violent  motion,  entirely  forgetting  that  if  it  exists  it  must 
be  a  natural  motion,  the  laws  of  which  are  altogether  different 
from  those  of  violent  motion.  Thus,  part  of  his  argument  was 
really  without  scientific  foundation,  though  iiis  conclusion  was 
correct.  Still,  Copernicus  did  about  all  that  could  have  been 
done  under  the  circumstances.  His  hypothesis  of  a  small  epi- 
cycle one-third  the  eccentricity  represented  the  m.otions  of  the 
planets  around  the  sun  with  all  the  exactness  that  observation 
then  admitted  of,  while,  in  the  absence  of  any  knowledge  of 
the  laws  of  motion,  it  was  impossible  to  frame  any  dynamical 
basis  for  the  motions  of  the  planets. 

§  2.   Ohliquitii  of  the  Ecliptic  ;  Seasons,  etc. ,'  on  the  Coper- 

nican  System. 

We  have  next  to  explain  the  relations  of  the  ecliptic  and 
equator  on  the  new  system.  Since,  on  this  system,  the  ce- 
lestial sphere  does  not  revolve  at  all,  what  is  the  significance 
of  the  pole  and  axis  around  which  it  seems  to  revolve?     The 


e  for  his  eccentricity,  and  g  for  the  mean  anomaly  of  thephxnet,  he  represented  its 
rectanguhir  coordinates  in  tiie  form 

X  =  a  (cos.  g  ~  e  ■\-  ^e  cos.  2^'), 

y  =  rt  (sin.  g  +  ^e  sin.  2g) ; 

while  the  approximate  modern  formula;  of  the  elliptic  motion  are — 

X  —  a  (cos.  y  —  a*  +  ie  cos.  2^'), 
y  =  a  (sin.  g  +  ^e  sin.  2g), 

which  agree  exactly  when  we  put  e  =  %e. 


G2       SYSTEM  OF  THE  WOULD  HISTORICALLY  DEVELOI'EI). 


answer  is,  tliat  the  celestial  poles  arc  the  points  among  the  stars 
towards  which  the  axis  of  the  earth  is  directed.  Here  the 
stars  are  sup})osed  to  be  infinitely  distant,  and  the  axis  of  the 
earth  to  be  continued  in  an  infinite  straight  line  to  meet  them. 
Since  this  point  appears  to  the  unassisted  sight  to  be  the  same 
during  the  entire  year,  it  follows  that  as  the  earth  moves  round 
the  sun^  its  axis  keeps  ])ointing  in  the  same  absolute  direction, 
as  will  be  shown  in  Fig.  18.  But  in  the  ])receding  chapter  we 
showed  that  there  is  a  slow  but  constant  change  in  the  position 
of  the  pole  among  the  stars,  called  precession,  which  the  an- 
cient astronomers  discovered  by  studying  observations  extend- 


FiG.  17. 


ing  through  several  centuries,  and  this  shows  that  on  the  Co- 
pernican  sj'stem  the  direction  of  the  earth's  axis  is  slowly 
changing. 

To  conceive  of  the  celestial  equator  on  the  Copernican  sys- 
tem, we  must  imagine  the  globular  earth  to  be  divided  into 
two  hemispheres  by  a  plane  intersecting  the  earth  around  its 
equator,  and  continued  out  on  all  sides  till  it  reaches  the  ce- 
lestial sphere.  This  may,  perhaps,  be  better  understood  by 
referring  to  Fig.  17,  representing  the  earth  in  the  centre  of  the 


OBLIQUITY  OF  THE  ECLIPTIC. 


63 


imaginary  celestial  sphere.  The  dotted  lines  passing  from  the 
poles  of  die  earth  to  tlie  points  P  and  S  mark  the  poles  of  that 
sphere.  It  is  evident  that  as  the  earth  turns  on  this  axis,  the 
celestial  sphere,  no  matter  how  great  it  may  seem  to  be,  will 
appear  to  turn  on  the  same  axis  in  the  opposite  direction. 
Again,  ej?  being  tlie  earth's  equator,  dividing  it  into  two  equal 
parts,  we  have  only  to  imagine  it  to  be  extended  to  J?  and  Q, 
all  round  the  celestial  sphere,  to  cut  the  latter  into  two  equal 
parts. 

Let  us  next  examine  more  closely  the  relation  of  the  earth 
to  the  sun.  We  have  already  shown  that  as  the  earth  moves 
around  the  sun,  the  latter  seems  to  move  around  the  celestial 
sphere,  and  the  circle  in  which  lie  seems  to  move  is  called  the 
ecliptic.  But  the  ecliptic  and  the  celestial  equator  are  in- 
clined to  each  other  by  an  angle  of  about  23^°.  This  shows 
that  the  axis  of  the  earth  is  not  perpendicular  to  its  orbit,  but 


FiQ.  18.— Causes  of  changes  of  seasons  ou  the  Coperuican  system. 

is  inclined  23^°  to  that  perpendicular,  as  shown  in  Fig.  18, 
which  represents  the  annual  course  of  the  earth  round  the 
sun.  It  is  of  necessity  drawn  on  a  very  incongruous  scale, 
because  tlie  distance  of  the  sun  from  the  earth  being  near- 
ly 12,000  diameters  of  the  latter  and  110  that  of  the  sun,  both 
bodies  would  be  almost  invisible  if  they  were  not  greatly  mag- 
nified in  the  figure.  A  difficulty  which  may  suggest  itself  is, 
that  the  present  figure  represents  the  earth  as  moving  away 


G-i     sysTi!:M  of  the  world  historically  developed. 

from  its  position  in  tlie  centre  of  tlie  sphere.  There  arc  two 
ways  of  avoiding  this  difficulty.  One  is  to  suppose  that  the 
observer  carries  the  Imaginary  celestial  sphere  with  him  as  he 
is  carried  around  the  sun ;  the  otlier  is  to  consider  the  sphere 
as  nearly  infinite  in  diameter.  The  latter  is  probably  the 
easiest  mode  of  conception  for  the  general  reader.  lie  must, 
therefore,  in  the  last  ligure  suppose  the  sphere  to  extend  out 
to  the  fixed  stars,  which  are  so  distant  that  the  whole  orbit  of 
the  earth  is  but  a  point  in  comparir^on  ;  and  the  different  points 
of  the  sphere  towards  which  the  poles  and  the  equator  of  the 
earth  point,  as  the  latter  moves  round  the  sun,  are  so  far  as  to 
appear  always  the  same.  It  now  requires  'but  an  elementary 
idea  of  the  geometry  of  the  sphere  to  sec  that  these  two  gi'eat 
circles  of  the  celestial  S})here — the  ecliptic,  around  which  the 
sun  seems  to  move,  and  the  equator,  which  is  everywhere 
equally  distant  from  the  points  in  which  tlie  earth's  axis  in- 
tersects tl?'  s])liere — will  appear  inclined  to  each  other  by  the 
same  angle  by  which  the  earth's  axis  deviates  from  the  per- 
pendicular to  the  ecliptic. 

Next,  we  have  to  see  how  the  clianges  of  the  seasons,  the 
piinoxcs,  etc.,  are  explained  on  the  Copernican  t]ieor3\  In 
the  last  figure  the  earth  is  represented  in  four  different  posi- 
tions of  its  annual  orl)it  around  the  sun.  In  the  position  A, 
the  soutli  pole  is  inclined  t^3^°  towards  the  sun,  while  the 
north  pole,  and  the  whole  region  withi';?  the  arctic  circle,  is 
enveloped  in  darkness.  Hence,  in  this  position,  the  sun  nei- 
ther rises  to  the  inhabitants  of  the  arctic  zone,  nor  sets  to 
those  of  the  antarctic  zone.  Outside  of  these  zones,  he  rises 
and  sets,  and  the  re..vti\c  mirths  of  dav  and  w'mac  at  anv 
place  can  be  estimated  by  Si..J.ying  the  circles  around  which 
that  place  is  carried  by  the  diurnal  turning  of  the  earth  on  its 
axis.  To  fac'litate  this,  we  j^rcsent  on  the  f<.)llowing  page  a 
magnified  picture  of  the  earth  at  A,  showing  more  fully  tlie 
iiemisphere  in  which  it  is  day  and  that  in  which  it  is  night. 
The  seven  nearly  liorizontal  lines  on  the  globe  arc  exain])les 
of  the  circles  in  question.  We  see  that  a  point  on  the  arctic 
circle  just  grazes  the  dividing-line  '  etween  light  and  d.irkn'jss 


THE  SEASONS. 


65 


once  in  its  revolution,  or  once  a  day;  that  is,  the  sun  just 
shows  himself  in  the  horizon  once  a  day.  Of  the  next  circle 
towards  the  south  about  two- 
thirds  is  in  the  dark,  and  one- 
third  in  the  light  hemisphere. 
This   shows  that  the  days  are 


about  twice  as  long  as  tlie 
nights.  This  circle  is  near  that 
"round  wliich  London  is  carried 
by  the  diurnal  revolution  of  the 
earth  on  its  axis.  As  we  go 
south,  we  see  that  tlie  propor- 
tion of  lii2;ht  on  the  diurnal  cir- 
cles  constantly  increases,  while  Fio.io._Eninit,a..i  view  ..f  tiie  emth  in 

J.    ■,      ■.  T      .     .   1  tliu  position  ^l  uf  llie  precutliiig  rti^'uru, 

that  or  darkness  dmnnishes,  un-  Rhowin-  winter  in  tiic  noitiiem  hemi- 
til  we  reach  the  equator,  where  'J''^"^'*^'  ''"'^  "*"'"'""■■ "'  ''^'^  «'»i"'""- 
they  are  equal.  When  we  pass  into  tlie  southern  hemisphere, 
we  see  the  light  covering  more  tlian  half  of  each  circle,  the 
proportion  of  light  to  darkness  constantly  increasing,  at  the 
same  rate  that  the  opposite  proportion  would  increase  in  going 
to  the  north.  Wlien  we  reach  the  antarctic  circle,  the  whole 
circle  is  in  the  light  hemisphere,  the  observer  just  grazing  the 
dividino;-line  at  midni<>;ht.  Inside  of  that  circle  the  observer 
is  in  sunlight  all  the  time,  so  that  the  sun  does  not  set  at  all. 
We  see,  then,  that  at  the  equator  the  days  and  nights  are  al- 
ways of  the  same  length,  and  that  the  inequrvlity  increases  as 
we  approach  cither  pole. 

We  now  go  on  three  months  to  the  position  ^,  which  the 
earth  occupies  in  ATarch,  Here  the  plane  of  the  terrestrial 
equator  being  continued,  passes  directly  through  the  suti  ;  the 
latter,  therefore,  seems  to  be  in  the  celestial  equator.  All  the 
diurnal  circles  arc  here  one-half  in  the  illuminated,  and  one- 
half  in  the  unilluminated  hemisphere,  the  latter  being  invisi- 
ble in  the  figure,  through  its  being  behind  the  earth.  The 
days  and  nights  are,  therefore,  of  equal  length  all  over  the 
globe,  if  we  call  it  night  whenever  the  sun  is  geometrically 
below  the  horizon.     In  the  position  6',  which  the  earth  takes 

6 


6Q       SYSTEM  OF  THE  WORLD  EISTOIUCALLY  DEVELOPED. 

in  June,  everything  is  the  same  as  in  position  A,  except  that 
effects  are  reversed  in  tlie  two  hemispheres.  The  northern 
hemisphere  now  has  tlie  longest  clays,  and  the  southern  one 
the  longest  nights.  At  D,  which  the  earth  reaches  in  Sep- 
tember, the  days  and  nights  are  equal  once  more,  for  tlie  same 
reason  as  in  B.  Thus,  all  the  seemingly  complicated  phenom- 
ena which  we  have  desci-ibed  in  the  preceding  chapter  are 
completely  explained  in  the  simplest  way  on  the  new  system. 
We  have  next  to  see  how  the  details  of  the  system  were  filled 
in  by  the  innnediate  successors  of  Copernicus. 

§  3.  Tycho  Bmhe. 

AVe  have  said  that  no  great  advance  could  be  made  upon 
the  Copernican  system,  without  either  a  better  knowledge  of 
the  laws  of  motion  or  more  exact  observations  of  the  positions 
of  the  heavenly  bodies.  It  was  in  the  latter  direction  that 
the  advance  was  first  made.  The  leader  was  Tvcho  Jiralie, 
who  was  born  in  154G,  three  years  after  the  death  of  Coperni- 
cus. Ilis  attention  M'as  first  directed  to  the  study  of  astron- 
omy by  an  eclipse  of  the  sun  on  August  21st,  15G0,  which  was 
total  in  some  parts  of  Europe.  Astonished  that  such  a  phe- 
nomenon could  be  predicted,  he  devoted  himself  to  a  study  of 
the  methods  of  observation  and  calculation  by  which  the  pre- 
diction was  made.  In  157G  the  King  of  Denmark  founded 
the  celebrated  Observatory  of  Uraniberg,  at  which  Tycho 
s{)ent  twenty  years,  assiduously  engaged  in  observations  of  the 
positions  of  the  heavenly  bodies  with  the  best  instruments  that 
could  then  be  made.  This  was  just  before  the  invention  of 
the  telescope,  so  that  the  astronomer  could  not  avail  himself 
of  that  powerful  instrument.  Consequently,  his  observations 
were  superseded  by  the  improved  ones  of  the  centuries  fol- 
lowing, and  their  celebrity  and  im]iortance  are  principally  due 
to  their  having  afforded  Kepler  the  means  of  discovering  his 
celebrated  laws  of  planetary  motion. 

As  a  theoretical  astronomer,  Tycho  was  unfortunate.  lie 
rejected  the  Copernican  system,  for  a  reason  which,  in  his  day, 
had  some  force,  namely,  the  incredible  distance  at  which  it 


TYCUO  BliAUE.  67 

was  necessary  to  suppose  the  fixed  stars  to  be  situated  if  that 
system  were  accepted.  We  have  shown  how,  on  the  Coperni- 
can  system,  the  outer  planets  seem  to  describe  an  annual  revo- 
hition  in  an  epicycle,  in  consequence  of  the  annual  revolution 
of  the  earth  around  the  sun.  The  fixed  stars,  which  are  sit- 
uated outside  the  solar  system,  must  a])pear  to  move  in  the 
same  way,  if  the  system  be  correct.  But  no  observations, 
whether  of  Tycho  or  his  predecessors,  had  shown  any  such 
motion.  To  this  the  friends  of  Copernicus  could  only  reply 
that  the  distance  of  the  fixed  stars  nuist  be  so  great  that  the 
motion  could  not  be  seen.  Since  a  vibration  of  three  or  four 
minutes  of  arc  might  have  been  detected  by  Tycho,  it  would 
be  necessary  to  suppose  the  stellar  sphere  at  least  a  thousand 
times  the  distance  of  the  sun,  and  a  hundred  times  that  of  Sat- 
urn, then  the  outermost  knoM'n  planet.  That  a  space  so  vast 
should  intervene  between  the  orbit  of  Saturn  and  the  fixed 
stars  seemed  entirely  incredible :  to  the  philosophers  of  the 
day  it  was  an  axiom  that  nature  would  not  permit  the  waste  of 
space  here  implied.  At  the  same  time,  the  proofs  given  by 
Copernicus  that  the  sun  was  the  centre  of  the  jdanetary  mo- 
tions were  too  strong  to  be  overthrown.  Tycho,  therefore, 
adopted  a  system  which  was  a  compound  of  the  Ptolemaic 
and  the  Copernican ;  he  supposed  the  five  planets  to  move 
around  the  sun  as  the  centre  of  their  motions,  while  the  sun 
was  itself  in  motion,  describing  an  animal  orbit  around  the 
earth,  which  remained  at  rest  in  the  centre  of  the  universe. 

Perhaps  it  is  fortunate  for  the  reception  of  the  Copernican 
system  that  the  astronomical  instruments  of  Tycho  were  not 
equal  to  those  of  the  beginning  of  the  present  century.  Had 
he  found  that  there  was  no  annual  parallax  among  the  stars 
amounting  to  a  second  of  arc,  and  therefore  that,  if  Coperni- 
cus was  right,  the  stars  must  be  at  leo  t  200,000  times  the  dis- 
tance of  the  sun,  the  astronomical  world  might  have  stood 
aghast  at  the  idea,  and  conc-luded  th;it,  after  all,  Ptolemy  must 
be  right,  and  Cojieriiicus  wrong. 

Tycho  never  elaborated  his  system,  and  it  is  hard  to  say 
how  he  would  have  answered  the  numerous  objections  to  it. 


()y     .■s\,sti:m  of  the  would  uistouically  developed. 

He  never  had  any  cHscii)los  of  eminence,  except  among  the 
ecclesiastics;  in  fact,  the  invention  of  the  telescope  did  away 
with  the  last  remaining  doubts  of  the  correctness  of  the  Co- 
pcrnican  system  before  a  new  one  M'ould  have  had  time  to 
gain  a  foothold. 

§  4.  I\ej)Ur. 

Ke})ler  was  born  in  1571,  in  Wurteniberg.  He  was  for  a 
while  the  assistant  of  Tycho  Brahe  in  his  calculations,  but  was 
too  clear-sighted  to  adopt  the  (;urious  system  of  his  nuister. 
Seeing  the  truth  of  the  Copernican  system,  he  set  himself  to 
determine  the  true  laws  of  the  motion  of  the  planets  around 
the  sun.  "We  have  seen  that  even  Copernicus  had  adopted  the 
ancient  theory,  that  all  the  celestial  motions  are  compounded 
(if  uniform  circular  motions,  and  had  thus  l)een  obliged  to  in- 
troduce a  small  e})icycle  to  account  for  the  irregularities  of 
the  motion.  The  observations  of  Tycho  were  so  much  more 
accurate  than  those  of  his  predecessoi-s,  that  they  sliowed  Kep- 
ler the  insufficiency  of  this  theory  to  represent  the  true  mo- 
tions of  the  planets  ound  the  sun.  The  }>lanet  most  favora- 
ble for  this  invest  gation  was  Mars,  being  at  the  same  time 
one  of  the  nearest  to  the  earth,  and  one  of  which  the  orbit 
was  most  eccentric.  The  only  way  in  which  Kepler  could 
])roceed  in  his  investigation  was  to  make  various  hypotheses 
lesnecting  the  orbit  in  which  the  planet  moved,  and  its  velocity 
in  various  points  of  its  orbit,  and  from  these  hy}>otheses  to  cal- 
culate the  positions  and  motions  of  the  i)lanet  as  seen  from 
the  earth,  and  then  compare  with  observations,  to  see  whether 
the  observed  and  calcidated  positions  agreed.  As  our  modern 
tables  of  logarithms  by  which  such  calculations  are  inmiensely 
abridged  were  not  then  in  existence,  each  trial  of  an  hypothe- 
sis cost  Kejdcr  an  imtnense  amount  of  labor.  Finding  that 
the  form  of  the  orbit  was  certainly  not  circular,  but  elliptical, 
he  was  led  to  try  the  effect  of  placing  the  sun  in  the  focus  of 
the  ellipse.  Then,  the  motion  of  the  j)lanet  would  be  satisfied 
if  its  velocity  were  made  variable,  beinc:  jxrcater  the  nearer 
it  M-as  to  the  sun.     Thus  he  was  at  leui^-th  led  to  the  first  two 


KEPLER. 


69 


of  his  three  celebrated  laws  of  planetary  motion,  which  arc  as 
follows : 

1.  The  orbit  of  each  jplanei  is  an  ell'qjse,  havhuj  the  sun  in 
one  focus. 

2.  As  the  planet  moves  I'ound  the  sun,  its  radius-vector  {or 
the  line  joining  it  to  the  sun)  passes  over  equal  areas  in 
equal  times. 

To  explain  these  laws,  let  PA  (Fig.  20)  be  the  ellipse  in 
which  the  planet  moves.    Then  the  sun  will  not  be  in  the  cen- 


Fio.  20.— Illustratiiitc  Kepler's  first  two  laws  of  planetary  motion. 

tre  of  the  ellipse,  Init  in  one  focus,  say  at  S,  the  other  focus 
being-  empty.  When  the  planet  is  at  J\\t  is  at  the  point  lear- 
est  the  sun;  this  point  is  therefore  called  the pe)'ihcl ion.  As 
it  passes  round  to  the  other  side  of  the  sun,  it  continues  to  re- 
cede from  liim  till  it  reaches  the  point  A,  when  it  attains  its 
greatest  distance.  This  point  is  the  aphelion.  Then  it  begins 
to  approach  the  sun  again,  and  continues  to  do  so  till  it  reaches 
Ponce  more,  when  it  again  i  gins  to  repeat  the  same  orbit. 
It  thus  describes  the  same  elliiise  over  and  over. 

Now,  suppose  that,  starting  from  J\  we  mark  the  position 
of  the  planet  in  its  orbit  at  the  end  of  any  equal  intervals  of 
time,  say  ;^0  days,  GO  days,  90  days,  120  days,  and  so  on.  Let 
a,  I),  c,  d  be  the  first  four  of  these  positions  between  each  of 
which  the  planet  has  required  30  days  to  move.  Di-aw  lines 
from  each  of  the  five  positions  of  the  i)lanet,  beginning  at  P., 


70        SYSTEM  OF  THE   WORLD  HISTORICALLY  DEVELOPED. 


to  the  sun  at  S.  We  shall  thus  have  four  triangular  spaces, 
over  each  of  which  the  radius-vector  of  the  planet  has  swept 
iu  30  days.  The  first  of  Kepler's  laws  means  that  the  areas 
of  all  of  these  spaces  will  be  equal. 

The  old  theory  that  the  motions  of  the  heavenly  bodies  must 
be  circular  and  uniform,  or,  at  least,  composed  of  circular  and 
uniform  motions,  was  thus  done  away  with  foi'ever.  The  el- 
lipse took  the  place  of  the  circle,  and  a  variable  motion  the 
place  of  a  uniform  one. 

Another  law  of  planetary  motion,  not  less  important  than 
these  two,  was  afterwards  discoAcred  by  Kepler.  Copernicus 
knew,  what  had  been  surmised  by  the  aticient  astronomers, 
that  the  more  distant  the  planet,  the  longer  it  took  it  to  per- 
form its  course  around  the  sun,  and  this  not  merely  because  it 
had  farther  to  go,  but  because  its  motion  was  I'eally  slower. 
For  instance,  Saturn  is  about  9|  times  as  far  as  the  earth,  and 
if  it  moved  as  fast  as  the  earth,  it  would  perform  its  revolu- 
tion in  9^  years ;  but  it  actually  requires  between  29  and  30 
.years.  It  does  not,  therefore,  move  one-third  so  fast  as  the 
earth,  although  it  has  nine  times  as  far  to  go.  Copernicus, 
however,  never  detected  any  relation  between  the  distances 
and  the  periods  of  revolution.  Kepler  found  it  to  be  as  fol- 
lows : 

Third  law  of  planetary  motion.  The  square  of  the  time 
of  revolution  of  each  2>hinet  is  projyortional  to  the  cube  of 
its  mean  distance  from  the  sun. 

This  law  is  shown  in  the  following  table,  which  gives  (1) 
the  mean  distance  of  each  })lanet  known  to  Kepler,  ex})rcssed 
in  astronomical  units,  each  unit  being  the  mean  distance  of 


Planets. 


(1) 
DiKtaiice. 


J/eims. .. 
'^iUtll  ... 

Afars 

Jii])iter.. 
iSntiirn  .., 


().;js7 

0. 72;i 

1. ()()() 

r).i'():{ 
!».r>:{i) 


(2) 

Cube  of  Diti- 

tiiiice. 


0..'i7S 
1. ()(>() 

:?.r)4() 

140.8 
8(18.0 


(3) 
(Years). 


O.LHl 

o.(;i.-> 

1. ()()() 

1.881 

11. 8(; 

2!).4(J 


(4) 

SquMre  of 
Period. 


O.OoS 
0.;{78 
1.001 

."..■);i8 
iio.do 

807.!) 


FROM  KEPLER   TO  NEWTON.  71 

the  earth  from  the  sun ;  (2)  the  cube  of  this  quantity ;  (3)  the 
time  of  revolution  in  years;  and  (4)  the  square  of  this  time. 

The  remarkable  agreement  between  the  second  and  fourth 
columns  will  be  noticed. 

§  5,  From  Kepler  to  Newton. 

So  far  as  the  determinali  'i  of  the  laws  of  planetary  motion 
from  observation  was  concerned,  we  might  almost  say  that 
Kepler  left  nothing  to  be  done.  Given  the  position  and 
magnitude  of  the  elliptic  orbit  in  which  any  planet  moved, 
and  the  point  of  the  orbit  in  which  it  was  found  at  any 
date,  and  it  became  possible  to  calculate  the  position  of  the 
planet  in  all  future  time.  More  than  that  science  could  not 
do.  It  is  true  that  the  places  of  the  planet  thus  predicted 
were  not  found  to  agree  exactly  with  observation ;  and  had 
Kepler  had  at  his  command  observations  as  accurate  as  those 
of  the  present  day,  lie  would  have  found  that  his  laws  could 
not  be  made  to  perfectly  represent  the  motion  of  the  planets. 
Not  oidy  would  the  elliptic  orbit  have  been  found  to  vary  its 
position  from  century  to  century,  but  the  planets  would  have 
been  found  to  dr-iate  from  it,  first  in  one  direction  and  then 
in  the  other,  w 'iie  the  areas  described  by  the  radius-vector 
would  have  been  sometimes  larger  and  sometimes  smaller. 
Why  should  a  planet  move  in  an  elliptic  orbit?  Why  should 
its  radius- vector  describe  areas  proportional  to  the  time? 
Why  should  there  be  that  exact  relation  between  their  dis- 
tances and  times  of  revolutions?  Until  these  questions  were 
answered,  it  would  have  been  impossible  to  say  why  the  plan- 
ets deviated  from  Kepler's  laws;  and  they  were  questions 
which  it  wjis  impossible  to  answer  until  tlic  general  laws  of 
motion,  unknown  in  Ke})lcr's  time,  were  fully  understood. 

The  first  important  step  in  the  discovery  of  tliese  laws  was 
taken  by  Galileo,  the  great  contemporary  of  Kepler,  one  of 
the  inventors  of  the  telescope,  and  the  first  Avho  ever  pointed 
that  instrument  >  t  the  heavens.  From  a  scientitic  ])oint  of 
view,  as  inventor  of  the  telescope,  founder  of  the  science  of 
dynamics,  teacher  and  upholder  of  the  Copernican  system,  and 


72       SYSTEM  OF  THE  WOULD  HISTORICALLY  DEVELOrED, 

sufferer  at  the  hands  of  the  Inquisition,  for  promulgating  vvliat 
he  knew  to  be  tlie  trutli,  Galileo  is  perhaps  the  most  interest- 
ing character  of  his  time.  If  any  serious  doubt  could  remain 
of  the  correctness  of  the  Copernican  system,  it  was  removed 
by  the  discoveries  made  by  the  telescoi)e.  The  phases  of 
Venus  showed  that  she  was  a  dark  globular  body,  like  the 
earth,  and  that  she  r«;ally  revolved  around  the  sun.  In  Jupi- 
ter and  his  satellites,  the  solar  system,  as  described  by  Coperni- 
cus, was  repeated  on  a  small  scale  with  a  fidelity  which  could 
not  fail  to  strike  the  thinking  observer.  There  was  no  longer 
any  opposition  to  the  new  doctrines  from  any  source  entitled 
to  respect.  The  Inquisition  forbade  their  promulgation  as 
absolute  truths,  but  were  perfectly  willing  that  they  should  be 
used  as  hypotheses ^  and  rather  encouraged  men  of  science  in 
the  idea  of  investigating  the  interesting  mathematical  prob- 
lems to  which  the  ex])lanation  of  the  celestial  motions  by  the 
Copernican  system  might  give  rise.  The  only  restriction  was 
that  they  must  stop  short  of  asserting  or  arguing  the  hypothe- 
ses to  be  a  reality.  As  this  assertion  was  implicitly  contained 
in  several  i)laces  in  the  great  work  of  Copernicus,  they  con- 
demned this  work  in  its  original  form,  and  ordered  its  revi- 
sion.* Probably  the  decree  of  the  Inquisition  was  entirely 
without  eifect  in  stopping  the  reception  of  the  Copernican 
system  outside  of  Italy  and  Spain. 

It  will  be  seen,  from  what  has  been  said,  that  the  next  step 
to  be  taken  in  the  direction  of  explaining  the  celestial  motions 
must  be  the  discovery  of  some  general  cause  of  those  motions, 
or,  at  least,  their  reduction  to  some  general  law.  Tlie  first 
attempt  to  do  this  was  made  l)y  Descartes  in  his  celebrated 
theor}'  of  vortices,  which  for  some  time  disputed  the  field  with 
Newton's  theory  of  gravitation.  This  philosopher  supposed 
the  sun  to  be  immersed  in  a  vast  mass  of  fluid,  extendin<>:  in- 
definitely  in  every  direction.     The  sun,  by  its  rotation,  set  the 

*  The  order  for  this  revision  was  nuule  at  the  time  of  condemninp;  Galileo's 
work,  hilt  I  am  not  aware  that  it  was  ever  executed.  An  edition  of  ("operniciis, 
ie\ised  to  satisfy  the  Inquisition,  would  certainly  he  an  interesting  work  to  the 
;i.-ti(iiiiiini(:il  hihiiopole  at  the  present  liine. 


FliOM  KEPLER   TO  NEWTON.  73 

parts  of  the  fluid  next  to  it  in  rotation ;  those  conununicated 
their  motions  to  the  parts  still  farther  ont,  and  so  on,  until 
the  whole  mass  was  set  in  rotation  like  a  whirlpool.  The 
planets  were  carried  around  in  this  ethereal  whirlpool.  The 
more  distant  planets  moved  more  slowly  because  the  ether 
was  less  affected  by  the  rotation  of  the  sun  the  more  distant 
it  was  from  him.  In  the  great  vortex  of  the  solar  system 
were  smaller  ones,  each  planet  being  the  centre  of  one ;  and 
thus  the  satellites,  floating  in  the  ether,  were  carried  round 
rheir  primaries.  Had  Descartes  been  able  to  show  that  the 
parts  of  his  vortex  must  move  in  elli[)ses  having  the  sun  in 
one  focus,  that  they  mnst  describe  ecpial  areas  in  equal  times, 
and  that  the  velocity  must  diminisli  as  we  recede  from  tlie 
sun,  according  to  Kepler's  third  law,  his  theory  would  so  far 
have  been  satisfactory.  Failing  in  this,  it  cannot  be  regarded 
as  an  advance  in  science,  but  rather  as  a  step  backwards.  Yet, 
the  great  eminence  of  the  philosopher  and  the  numl)er  of  his 
disciples  secured  a  wide  currency  for  his  theory,  and  we  find 
it  supported  by  no  less  an  authority  than  John  J3ernoulli. 

After  Galileo,  the  man  who,  ]ierhaps,  did  most  to  prejiare 
the  way  for  gravitation  w^as  lluyghens.  As  a  mathematician, 
a  mechanician,  and  an  observer,  he  stood  in  the  first  rank. 
lie  discovei'cd  the  laws  of  centrifugal  force,  and  if  he  had 
simply  applied  these  laws  to  the  solar  system,  he  would  have 
been  led  to  the  result  tliat  the  planets  arc  held  in  their  orbits 
by  a  force  varying  as  the  inverse  square  of  their  distance  from 
the  sun.  Having  found  this,  the  road  to  the  theory  of  gravita- 
tion could  hardly  ha^e  been  missed.  But  the  great  discovery 
seemed  to  require  a  mind  freshly  formed  for  the  occasion. 


74       SYSTEM  OF  THE  WOULD  HISTOllICALLY  DEVELOPED. 


CHAPTER  III. 

UNIVERSAL    GKAVITATION. 

§  1.  Newton. — Discovery  of  Oravitation. 

Tin.  real  Rigiiificjince  of  Newton's  great  discovery  of  univer- 
sal gravitation  is  fully  appreciated  by  but  few.  Gravitation 
is  generally  thought  of  as  a  mysterious  force,  acting  only  be- 
tween the  heavenly  bodies,  and  first  discovered  by  Newton. 
Had  gravitation  itself  been  discovered  by  Newton  as  some 
new  principle  to  account  for  the  motions  of  the  planets,  it 
would  not  have  been  so  admirable  a  discovery  as  that  which 
he  actually  made.  Gravitation,  in  a  somewhat  limited  s})here, 
is  known  to  all  men.  It  is  simply  the  force  which  causes 
all  heavy  bodies  to  fall,  or  to  tend  towards  the  centre  of  the 
earth.  Every  one  who  had  ever  seen  a  stone  fall,  or  felt  it  to 
be  heav}',  knew"  of  the  existence  of  gravitalion.  What  New- 
ton did  was  to  show  that  the  motions  of  the  planets  were 
determined  by  a  universal  force,  of  Avhich  the  foi'ce  which 
caused  the  a))ple  to  fall  was  one  of  the  manifestations,  and 
thus  to  deprive  the  celestial  motions  of  all  the  mystery  in 
which  they  had  formerly  been  enshrouded.  To  his  predeces- 
sors, the  continuous  motion  of  the  ])lanets  in  circles  or  ellipses 
was  something  so  completely  unlike  any  motion  seen  on  the 
surface  of  the  earth,  that  thev  (iould  not  imaijino  it  to  be  ffov- 
erned  by  the  same  laws;  and,  knowing  of  no  law  to  limit  the 
planetary  motions,  the  idea  of  the  heavenly  bodies  moving  in 
a  manner  which  set  all  the  laws  of  terrestrial  motion  at  de- 
fiance was  to  them  in  no  wav  incredible. 

The  idea  of  a  cosmical  force  emanating  from  the  sun  or  the 
earth,  and  causing  the  (celestial  motions,  did  not  originate  with 
Newton.  We  have  seen  that  even  Ptolemy  had  an  idea  of  a 
force  which,  always  directed  towards  the  centre  of  the  earth, 


NEWTON.— DISCOVERY  OF  GRAVITATION.  Y5 

or,  wliicli  was  to  him  the  same  thiiij^,  towards  tlie  centre  of 
the  imiverso,  not  only  caused  heavy  bodies  to  fall,  but  bound 
the  whole  universe  tofj^ether.  Kepler  also  maintained  that  the 
force  which  moved  the  planets  resided  in,  and  emanated  from, 
the  sun.  But  neither  Ptolemy  nor  Kepler  could  give  any  ade- 
quate explanation  of  the  force  on  the  basis  of  laws  seen  in  ac- 
tion around  us;  nor  was  it  possible  to  form  any  conception  of  its 
true  nature  without  a  knowledge  of  the  general  laws  of  motion 
and  force,  to  which  neither  of  these  philosophers  ever  attained. 

The  great  misap[)rehensio!i  which  possessed  the  minds  of 
nearly  all  mankind  till  the  time  of  Galileo  was,  that  the  con- 
tinuous action  of  some  force  was  necessary  to  keep  a  moving 
body  in  motion.  That  Kepler  himself  was  fully  possessed  of 
this  notion  is  shown  by  the  fact  that  he  conceived  a  force  act- 
ing only  in  the  direction  of  the  sun  to  be  insuihcient  for  keep- 
ing up  the  planetary  motions,  and  to  require  to  be  supplement- 
ed by  some  force  which  should  constantly  push  the  planet 
ahead.  The  latter  force,  he  conceived,  might  arise  from  the 
rotation  of  the  sun  on  his  axis.  It  is  hard  to  say  who  was  the 
first  clearly  to  see  and  announce  that  this  notion  was  entirely 
incorrect,  and  that  a  bodv  once  set  in  motion,  and  acted  on  bv 
no  force,  would  move  forwards  forever  —  so  gradually  did  the 
great  truth  dawn  on  the  minds  of  men.  It  must  have  been 
obvious  to  Leonardo  da  Vinci;  it  was  implicitly  contained  in 
Galileo's  law  of  fallino;  bodies,  and  in  IIuv<>;hens's  theory  of 
central  forces;  yet  neither  of  these  philosophers  seems  to  have 
clearly  and  completely  expressed  it.  We  can  hardly  be  far 
wrong  in  saviniif  that  Newton  was  the  first  who  clearly  laid 
down  this  law  in  comiection  with  the  correlated  laws  which 
cluster  around  it.  The  basis  of  Newton's  discovery  were  these 
three  laws  of  motion  : 

First  law.  A  hod//  onre  set  in  motion  and  acted  on  Jnj  no  force 
icill  move  forwards  in  a  straight  line  and  with  a  uniform  velocity 
forever. 

Second  law.  If  a  nwvin;/  body  he  acted  on  hy  any  force,  its  de- 
viation from  the  motion  defined  in  the  first  law  will  he  in  the  direc- 
tion of  die  force ^  and  proportioned  to  it. 


108  PRACTICAL  ASTRONOMY. 

was  in  Venice  on  a  visit,  and  there  received  a  letter  from 
Paris,  in  which  the  invention  was  mentioned.  lie  at  once  set 
himself  to  the  reinvention  of  the  instrnraent,  and  was  so  suc- 
cessful that  in  a  few  days  he  exhibited  a  telescope  magnify- 
ing three  times,  to  the  astonished  authorities  of  the  city.  Re- 
turning to  liis  home  in  Florence,  he  made  other  and  larger 
ones,  which  revealed  to  him  the  spots  on  the  sun,  the  phases 
of  Venus,  the  mountains  on  the  moon,  the  satellites  of  Jupiter, 
the  seeming  handles  of  Saturn,  and  some  of  the  myriads  of 
stars,  separately  invisible  to  the  naked  eye,  whose  combined 
light  forms  the  milky-way.  But  the  lai-gest  of  these  instru- 
ments magnified  only  about  thirty  times,  and  was  so  imper- 
fect in  construction  as  to  be  far  from  showing  as  much  as 
could  be  seen  with  a  modern  telescope  of  that  power.  The 
Galilean  telescope  was,  in  fact,  of  the  simplest  construction, 
consisting  of  the  combination  of  a  pair  of  lenses,  of  which  the 
larger  was  convex  and  the  smaller  concave,  as  shown  in  the 
following  figure : 


Fio.  28.— The  Galilean  telescope.    The  dotted  lines  show  the  course  of  the  rays  through 

the  lenses. 

The  distance  of  the  lenses  was  such  that  the  rays  of  light 
from  a  star  passing  through  the  large  convex  lens,  or  object- 
glass,  OB,  met  the  concave  lens,  R,  before  reaching  the  focus. 
The  position  of  this  concave  lens  was  such  that  the  rays 
should  emerge  from  it  nearly  parallel.  This  form  of  tele- 
scope is  still  used  in  opera -glasses,  because  it  can  be  made 
shorter  than  any  other. 

The  improvements  in  the  telescope  since  Galileo  can  be 
best  understood  if  we  give  a  brief  statement  of  the  princi- 
ples on  which  all  modern  telescopes  are  constructed.  The 
properties  of  every  such  instrument  depend  on  the  power  pos- 
sessed by  a  lens  or  by  a  concave  mirror  of  forming  an  im- 
age of  any  distant  object  in  its  focus.     Thid  is  done  in  the 


118  PRACTICAL  ASTRONOMY. 

The  eye-piece  of  a  telescope,  as  well  as  its  objective,  con- 
sists of  two  glasses.  A  single  lens  will,  indeed,  answer  all 
the  purposes  of  seeing  an  object  in  the  centre  of  the  field 
of  view,  but  the  field  itself  will  be  narrow  and  indistinct  at 

the  edges.  An  additional  lens,  term- 
ed tlie  field -lens,  is  therefore  placed 
very  near  the  image,  for  the  purpose 
of  refracting  the  outer  rays  into  the 
proper  direction  to  form  a  distinct 
image  with  the   aid  of  the  eye -lens. 

'""■■  "TrT^Lt:'"'"""  I"  'l''ig-  33  Buch  an  eye-piece  is  rep- 

resented,  in  which  the  field -lens  is 
between  the  image  and  the  eye.  This  is  called  a  posiiive 
eye-piece.  In  the  negative  eye -piece  the  rays  pass  through 
the  field-lens  just  before  coming  to  a  focus,  so  that  the  image 
is  formed  just  within  that  lens.  The  positive  eye -piece  is 
used  when  it  is  required  to  use  a  micrometer  in  the  focal 
plane  ;  but  for  mere  looking  the  negative  ocular  is  best.  All 
telescopes  are  supplied  with  a  number  of  eye -pieces,  by 
changing  which  tlie  magnifying  power  may  be  altered  to  suit 
the  observer. 

The  astronomical  telescope  used  with  these  eye-pieces  al- 
ways shows  objects  upside  down  and  right  side  left.  This 
causes  no  inconvenience  in  celestial  observations.  But  for 
viewing  terrestrial  objects  the  eye-piece  must  have  two  pairs 
of  lenses,  the  first  of  which  forms  a  new  image  of  the  object 
restored  to  its  proper  position,  which  image  is  viewed  by  the 
eye -piece  formed  of  the  second  pair.  This  combination  is 
called  an  erecting  or  terrestrial  eye-piece. 

§  3.  The  Mounting  of  the  Telescope. 

If  the  earth  did  not  revolve,  so  that  each  heavenly  body 
would  be  seen  hour  after  hour  and  day  after  day  in  nearly 
tlie  same  direction,  the  problem  of  using  great  telescopes 
would  be  much  simplified.  The  objective  and  the  eye-piece 
could  be  fixed  so  as  to  point  at  the  object,  and  the  observer 
could  scrutinize  it  at  his  leisure.     But  actually,  when  we  use 


THE  MOUNTING    OF  THE  TELESCOPE. 


119 


a  telescope,  the  diurnal  revolution  of  the  earth  is  apparently 
increased  in  proportion  to  the  magnifying  power  of  the  in- 
strument ;  and  if  the  latter  is  fixed,  and  a  high  power  is  used, 
the  object  passes  by  with  such  rapidity  that  it  is  impossible  to 
scrutinize  it.  Merely  to  point  a  telescope  at  an  object  needs 
many  special  contrivances,  because,  unless  the  pointing  is  ac- 
curate, the  object  cannot  be  found  at  all.  With  a  telescope, 
and  nothing  more,  an  observer  might  spend  half  an  hour  in 
vain  efforts  to  point  it  at  Sirius  so  accurately  that  the  image 
of  the  star  should  be  brought  into  the  field  of  view ;  and  then, 
before  he  got  one  good  look,  it  might  flit  away  and  be  lost 
again.  If  this  is  the  case  with  a  bright  star,  how  much  liarder 
must  it  be  to  point  at  the  planet  Neptune,  an  object  invisible 
to  the  naked  eye,  which  is  not  in  the  same  direction  two  min- 
utes in  succession !  It  will  readily  be  understood  that,  to  make 
any  astronomical  use  of  a  large  telescope,  two  things  are  abso- 
lutely necessary :  first,  the  means  of  pointing  the  telescope  at 
any  object,  visible  or  invisible ;  and,  second,  the  means  of  mov- 
ing the  telescope  so  that 
it  shall  follow  the  object 
in  its  diurnal  motion, 
and  thus  keep  its  image 
in  the  field  of  view.  The 
following  are  the  me- 
chanical contrivances  by 
which  these  objects  are 
effected : 

The  object-glass  is 
placed  in  one  end  of  a 
tube,  OE,  the  length  of 
the  tube  being  nearly 
equal  to  the  focal  length 
of  the  objective.  The 
eye-piece  is  fitted  into  a 
projection  at  the  lower 
end  of  the  tube,  K.    The 

1  •      1.      f  4.1        4.    1        •     i.      ^'*'*  84.— Modi!  (if  mounting  a  telescope  so  as  to  fol- 
Object  or  the    tube    is  to  low  a  star  in  Us  dlunml  motion. 


120  PRACTICAL  ASTRONOMY. 

keep  the  glasses  in  their  proper  relative  positions,  and  to  pro- 
tect the  eye  of  the  observer  from  stray  light. 

The  tube  has  an  axis,  ^/i,  firmly  fastened  to  it  at  A  near  its 
middle,  which  axis  passes  through  a  cylindrical  case,  C,  into 
which  it  neatly  fits,  and  in  which  it  can  turn.  By  turning  the 
telescope  on  this  axis,  the  end  E  can  be  brought  towards  the 
reader,  and  0  from  him,  or  vice  versa.  This  axis  is  called  the 
declination  axis.  The  case,  C,  is  firmly  fastened  to  a  second 
axis,  DE,  supported  at  D  and  E  called  the  2^ol(ir  axis.  This 
axis  points  to  the  pole  of  the  heavens,  and,  by  turning  it,  the 
whole  telescope,  with  the  part,  A  C,  of  the  case,  may  be  brought 
towards  the  observer,  while  the  end  B  will  recede  from  him, 
or  vice  versa.  In  order  that  the  weight  of  the  telescope  may 
not  make  it  turn  on  the  polar  axis,  it  is  balanced  by  a  weight 
at  Bj  on  the  other  end  of  the  declination  axis.  This  weight 
is  commonly  divided,  a  part  being  carried  by  the  axis,  and  a 
part  by  the  case,  C.  The  polar  axis  is  carried  by  a  frame,  F, 
well  fastened  on  top  of  a  pier  of  masonry. 

Such  is  the  general  nature  of  the  mechanism  by  which  an 
astronomical  telescope  is  mounted.  The  essential  point  is 
that  there  shall  be  two  axes — one  fixed,  and  pointing  at  the 
pole,  and  one  at  right  angles  to  it,  and  turning  with  it.  In 
tlie  arrangement  of  these  axes  there  are  great  differences  in 
the  telescopes  of  different  makers;  but  Fig.  34  shows  what 
is  essential  in  the  plan  of  mounting  now  very  generally 
adopted. 

In  the  figure  the  telescope  is  represented  as  east  of  the  spec- 
tator, and  as  pointed  at  the  pole,  and  therefore  parallel  to  the 
polar  axis.  Suppose  now  that  the  telescope  be  turned  on  the 
declination  axis,  AB,  through  an  arc  of  90°,  the  eye-piece,  E^ 
being  brought  towards  the  spectator ;  the  object  end  will  then 
point  towards  the  east  horizon,  and  therefore  towards  the  celes- 
tial equator,  the  eye  end  pointing  directly  towards  the  spec- 
tator. Then  let  the  whole  instrument  be  turned  on  the  polar 
axis,  the  eye-piece  being  brought  downwards.  The  telescope 
will  then  move  along  the  celestial  equator,  or  the  path  of  a 
star,  90°  from  the  pole.     And  at  whatever  distance  from  the 


THE  REFLECTING    TELESCOPE.  121 

pole  we  set  it  by  turning  it  on  the  declination  axis,  if  we 
turn  it  on  the  polar  axis  it  will  describe  a  circle  having  the 
pole  at  its  centre ;  that  is,  the  same  circle  which  a  star  follows 
by  its  diurnal  motion.  So,  to  observe  a  star  with  the  telescope, 
we  have  first  to  turn  it  on  the  declination  axis  to  the  polar  dis- 
tance of  the  star,  and  then  on  the  polar  axis  till  it  points  at 
the  star.  This  pointing  is  effected  by  circles  divided  into  de- 
grees and  minutes,  not  shown  in  the  figure,  by  which  the  dis- 
tance which  the  telescope  points  from  the  pole  and  from  the 
meridian  may  be  found  at  any  time. 

In  order  that  the  star,  when  once  found,  may  be  kept  in  the 
field  of  view,  the  telescope  is  furnished  with  a  system  of  clock- 
work, by  which  the  polar  axis  is  slowly  turned  at  the  rate  of 
one  revolution  a  day.  By  starting  this  clock-work,  the  tele- 
scope is  made  to  follow  the  star  in  its  diurnal  motion  ;  or,  to 
speak  with  greater  astronomical  precision,  as  the  earth  turns 
on  its  axis  from  west  to  east,  the  telescope  turns  from  east  to 
west  with  the  same  angular  velocity,  so  that  the  direction  in 
which  it  points  in  the  heavens  remains  unaltered. 

In  order  to  facilitate  the  finding  or  recognition  of  an  object, 
the  telescope  is  furnished  with  a  "  finder,"  2\  consisting  of  a 
small  telescope  of  low  power  pointing  in  the  same  direction 
with  the  larger  one.  An  object  can  be  seen  in  the  small  tel- 
escope without  the  pointing  being  so  accurate  as  is  necessary 
in  the  case  of  the  large  one  ;  and,  when  once  seen,  the  tele- 
scope is  moved  until  the  object  is  in  the  middle  of  the  field 
of  view,  when  it  is  also  in  the  field  of  view  of  the  large  one. 

§  4.   The  Refiectinfj  Tdescope. 

Two  radically  different  kinds  of  telescopes  are  made :  the 
one  just  described,  known  as  the  refracting  telescope,  because 
dependent  on  the  refraction  of  light  through  glass  lenses ;  and 
the  other,  the  reflecting  telescope,  so  called  because  it  acts  by 
reflecting  the  light  from  a  concave  mirror.  The  name  of  the 
first  inventor  of  this  instrument  is  disputed ;  but  Sir  Isaac 
Newton  was  among  the  first  to  introduce  it.  It  was  designed 
by  him  to  avoid  the  difficulty  growing  out  of  the  chromatic 


122  PRACTICAL  ASTRONOMY. 

aberration  of  the  refracting  telescopes  of  his  time,  wliich,  it 
will  be  reiuembered,  were  not  achromatic.  If  parallel  rays  of 
light  from  a  distant  object  fall  upon  a  concave  mirror,  as  shown 
in  Fig.  35,  they  will  all  be  reflected  back  to  a  focus,  l\  half- 
way between  the  centre  of  curvature,  G,  and  the  surface  of 


:!Ee:-:jt. 


Fio.  35.— Speculum  brlngiiifj  rays  to  a  single  focus  by  reflection. 

the  mirror.  In  order  that  the  rays  may  be  all  reflected  to 
absolutely  the  same  focus,  the  section  of  the  mirror  must  be 
a  parabola,  and  the  point  where  the  rays  meet  will  be  the 
focus  of  the  parabola.  If  the  rays  emanate  from  the  various 
points  of  an  object,  an  image  of  this  object  will  be  formed 
in  and  near  the  focus,  as  in  the  case  of  a  lens.  This  image 
is  to  be  viewed  with  a  magnifying  eye-piece  like  that  of  a 
retracting  telescope.     Such  a  mirror  is  called  a  speculum. 

Here,  however,  a  difficulty  arises.  The  image  is  formed  on 
the  same  side  of  the  mirror  on  which  the  object  lies;  and  in  or- 
der that  it  may  be  seen  directly,  the  eye  of  the  observer  and 
the  eye-piece  must  be  between  F  and  0,  directly  in  the  rays 
of  light  emanating  from  the  object.  By  placing  the  eye  here, 
not  only  would  a  great  deal  of  the  light  be  cut  off  by  the  body 
of  the  observer,  but  the  definition  of  the  image  would  be  great- 
ly injured  by  the  interposition  of  so  large  an  object.  Three 
plans  have  been  devised  for  evading  this  difficulty,  wliich  are 
due,  respectively,  to  Gregory,  Newton,  and  Ilerschel. 

The  Herschelian  Telescope.  —  In  this  form  of  telescope  the 
mirror  is  slightly  tipped,  so  that  the  image,  instead  of  being 
formed  in  the  centre  of  the  tube,  is  formed  near  one  side  of 
it,  as  in  Fig.  36.  The  observer  can  then  view  it  without  put- 
ting his  head  inside  the  tube,  and,  therefore,  without  cutting 
off  any  material  portion  of  the  light.  In  observation,  he  must 
stand  at  the  upper,  or  outer,  end  of  the  tube,  and  look  into  it, 
his  back  being  turned  towards  the  object.     From  his  looking 


THE  REFLECTING   TELESCOPE. 


123 


directly  into  the  mirror,  it  was  also  called  tlie  "  front-view  " 
telescope.     The  great  disadvantage  of  this  arrangement  is  that 


Fig.  36.— Ilerschelinn  telescope. 

the  rays  cannot  be  Lroiiglit  to  an  exact  focus  when  they  are 
thrown  so  far  to  one  side  of  the  axis,  and  the  injury  to  the 
deiinition  is  so  gi'eat  that  the  front-view  plan  is  now  entirely 
abandoned. 

The  Newtonian  Telescope. — The  plan  proposed  by  Sir  Isaac 
Newton  was  to  place  a  small  plane  mirror  just  inside  the  fo- 
cus, inclined  to  the  telescope  at  an  angle  of  45°,  so  as  to  throw 
the  rays  to  the  side  of  tiie  tube,  where  they  come  to  a  focus, 
and  form  the  image.  An  opening  is  made  in  the  side  of  the 
tube,  just  below  where  the  image  is  formed  in  which  the  eye- 
piece is  inserted.  Tliis  mirror  cuts  off  some  of  the  light,  but 
not  enough  to  be  a  serious  defect.  An  improvement  which 
lessens  this  defect  has  been  made  by  Professor  Henry  Draper. 


Fio.  37.— Horizontal  section  of  a  Newtonian  telescope.  This  section  shows  how  the  lumi- 
nous rays  reflected  from  the  parabolic  mirror  M  meet  a  small  rectangular  prism  w  n, 
which  replaces  the  inclined  plane  mirror  used  in  the  old  form  of  Newtonian  telescope. 
After  undergoing  a  total  reflection  from  m  n,  the  rays  form  at  a  6  a  very  small  image 
of  the  heavenly  body. 

The  inclined  mirror  is  repl-aced  b}'  a  small  rectangular  prism, 
by  reflection  from  which  the  imago  is  formed  very  near  the 
prism.     A  pair  of  lenses  arc  then  inserted  in  the  course  of 


124  PRACTICAL  ASTRONOMY. 

the  rays,  by  which  a  second  image  is  formed  at  the  opening 
in  the  side  of  the  tube,  and  tliis  second  image  is  viewed  by 
an  ordinary  eye -piece.  The  fom*  lenses  togetlier  form  an 
erecting  eye-piece. 

The  Gnyorian  Telescope. — Tiiis  is  a  form  proposed  by  James 
Gregory,  who  probably  preceded  Newton  as  an  inventor  of  the 
reflecting  telescope.  Behind  the  focus,  F.,  a  small  concave 
mirror,  R^  is  placed,  by  which  the  light  is  reflected  back  again 


Fio.  38.— Section  of  the  Gregoriau  telescope. 

down  the  tube.  The  larger  mirror,  M,  has  an  opening  through 
its  centre,  and  the  small  mirror,  i?,  is  so  adjusted  as  to  form  a 
second  image  of  the  object  in  this  opening.  This  imnge  is 
then  viewed  by  an  eye-piece  which  is  screwed  into  the  opening. 

The  Cassegrainian  Telescope — In  principle  the  same  with  the 
Gregorian,  diffei's  from  it  only  in  that  the  small  mirror,  R,  is 
convex,  and  is  placed  inside  the  focns,  F,  so  that  the  rays  are 
reflected  from  it  before  reaching  the  focus,  and  no  image  is 
formed  until  they  reach  the  opening  in  the  large  mirror. 
This  form  has  an  pdvantas-e  over  the  Gregorian  in  that  the 
telescope  may  be  made  shorter,  and  the  small  mirror  can  be 
more  easily  shaped  to  the  required  figure.  It  has  therefore 
entirely  superseded  the  original  Gregorian  form. 

Optically,  these  forms  of  telescope  are  inferior  to  tlie  New- 
tonian. But  the  latter  is  subject  to  the  inconvenience  that  the 
observer  must  be  stationed  at  the  upper  end  of  the  telescope, 
where  he  looks  into  an  eye-piece  screwed  into  the  side  of  the 
tube.  If  the  t^^lescope  is  a  small  one,  this  inconvenience  is 
not  felt ;  but  with  large  telescopes,  twenty  feet  long  or  up- 
wards, the  case  is  entirely  different.  Means  must  tlien  be  pro- 
vided by  which  the  observer  may  be  carried  in  the  air  at  a 
height  equal  to  the  length  of  the  instrument,  and  this  requires 
considerable  mechanism,  the  management  of  which  is  often 


THE  PRINCIPAL   TELESCOPES  OF  MODERN  TIMES.     125 

very  troublesome.  On  the  other  hand,  the  Cassegrainian  tele- 
scope is  pointed  directly  at  the  object  to  be  viewed,  like  a  re- 
fractor, and  the  observer  stands  at  the  lower  end,  and  looks  in 
at  the  opening  through  the  large  mirror.  This  is,  therefore, 
the  most  convenient  form  of  all  in  management.  One  draw- 
back is,  that  there  are  two  mirrors  to  be  looked  after,  and,  un- 
less the  figure  of  both  is  perfect,  the  image  will  be  distorted. 
Another  is  the  great  size  oi  the  image,  which  forces  the  ob- 
server to  use  either  a  higii  magnifying  power,  or  an  eye-piece 
of  corresponding  size.*  But  these  defects  are  of  little  impor- 
tance compared  with  the  great  advantage  of  convenient  use. 

§  5.  The  Principal  Great  Reflecting  Telescopes  of  Modern  Times. 

The  reflecting  telescopes  made  by  Newton  and  his  contem- 
poraries were  very  small  indeed,  none  being  more  than  a  few 
inches  in  diameter.  Tliouc-h  vastlv  more  manageable  than  the 
immensely  long  refractors  of  Iluyghens,  they  do  not  seem  to 
have  exceeded  them  in  effectiveness.  We  might,  therefore, 
have  expected  the  achromatic  telescope  to  supersede  the  re- 
flector entirely,  if  it  could  be  made  of  large  size.  But  in  the 
time  of  Dollond  it  was  impossible  to  produce  disks  of  flint-glass 
of  suflicient  uniformity  for  a  telescope  more  than  a  very  few 
inches  in  diameter.  An  achromatic  of  four  inches  aperture 
was  then  considered  of  extraordinary  size,  and  good  ones  of 
more  than  two  oi  three  inches  were  rare.  Consequently,  for 
the  purpose  of  seeing  the  most  faint  and  difticult  objects,  the 
earlier  achromatics  were  little,  if  anj',  better  than  the  long 
telescopes  of  Iluyghens  and  Cassini.  As  there  were  no  such 
obstacles  to  the  polishing  of  large  mirrors,  it  was  clear  that  it 
was  to  the  reflecting  telescope  that  recourse  must  be  had  for 
any  great  increase  in  optical  power.  Before  the  middle  of 
the  last  century  the  reflectors  w^ere  little  larger  than  the  re- 
fractors, and  had  not  exceeded  them  in  their  optical  perform- 
ance. But  a  genius  now  arose  who  was  to  make  a  wonderful 
improvement  in  their  construction. 

*  The  Melbourne  telescope  has  nn  eye-lens  six  inches  in  diameter. 


12C  PRACTICAL  ASTRONOMY. 

William  Ilersclicl,  in  17CG,was  a  churcli-organist  and  teach- 
er of  music  of  very  high  repute  in  Bath,  who  spent  what  little 
leisure  he  had  in  the  study  of  mathematics,  astronomy,  and 
oj)tics.  J>y  accident  a  Gregorian  reflector  two  feet  long  f(  11 
into  his  hands,  and,  turning  it  to  the  heavens, he  was  so  enrapt- 
ured with  the  views  presented  to  him  that  he  sent  to  London 
to  see  if  he  could  not  purchase  one  of  greater  ])ower.  The 
price  named  being  far  above  his  means,  he  resolved  to  make 
one  for  himself.  After  numy  experiments  with  metallic  al- 
loys, to  learn  which  Avould  reflect  most  light,  and  many  efforts 
to  find  the  best  way  of  polishing  his  mirror,  and  giving  it  a 
parabolic  form,  he  produced  a  five-foot  Newtonian  reflector, 
which  revealed  to  him  a  number  of  interesting  celestial  phe- 
nomena, though,  of  course,  nothing  that  was  not  already  known. 
Determined  to  aim  at  nothing  less  than  the  largest  telescope 
that  could  be  made,  he  attempted  vast  numbers  of  mirrors  of 
constantly  increasing  size.  The  large  majority  of  the  individ- 
ual attempts  were  failures ;  but  among  the  results  of  the  suc- 
cessful attempts  were  telescopes  of  constantly  increasing  size, 
until  he  attained  the  hitherto  unthouglit-of  aperture  of  two  feet, 
with  a  length  of  twenty  feet.  With  one  of  these  he  discov- 
ered the  planet  Uranus.  The  fame  of  the  nnisician-astrono- 
mer  reaching  the  ears  of  King  George  III.,  that  monarch  gave 
him  a  pension  of  £200  per  annum,  to  enable  him  to  devote 
his  life  to  a  career  of  astronomical  discovery.  He  now  made 
the  greatest  stride  of  all  by  completing  a  reflector  four  feet 
in  diameter  and  forty  feet  long,  with  which  he  discovered  two 
new  satellites  of  Saturn. 

Ilerschel  now  found  that  he  had  attained  the  limit  of  man- 
ageable size.  The  observer  had  to  be  suspended  perhaps  thir- 
ty or  forty  feet  in  the  air,  in  a  room  large  enough  to  hold,  not 
only  himself,  but  all  the  means  necessary  for  recording  his 
observations ;  and  this  room  had  to  follow  the  telescope  as  it 
moved,  to  keep  a  star  in  the  field.  To  this  was  added  the 
difficulty  of  keeping  the  mirror  in  proper  figure,  the  mere 
change  of  temperature  in  the  night  operating  injuriously  in 
this  respect.     We  need  not,  therefore,  be  surprised  to  learn 


THE  riilNCIVAL  TELESCOPES  OF  MODERN  TIMES.     T27 


f  10.  39.— Herschel's  great  telescope. 

that  Tlerschel  made  very  little  use  of  this  instrument,  and  pre- 
ferred the  twenty-foot  even  in  scrutinizing  the  most  difficult 
objects.* 

*  Herschel's  great  instrument  is  still  preserved,  but  is  not  mounted  i  ;i  ^^e; 
indeed,  it  is  probable  that  the  mirror  lost  all  its  lustre  long  years  ago.  ^r  i  "J'', 
Sir  John  Ilerschel  dismounted  it,  laid  it  in  a  horizontal  positic.i,  and  closed  it  'i> 
after  a  family  celebration  inside  the  tube,  at  which  the  following  iong  was  sunf  : 

THE  OLD  TELESCOPE. 

[To  he  sung  on  Ncie-ycar's-eve,  lS3f>-'40,  hy  Papa,  Mamma,  Madame  Gerlach,  and  all  the  Little 

Bwiica  in  the  Tube  thereof  assembled.) 

In  the  old  Telescope's  tube  we  sit, 
And  the  shades  of  the  past  around  us  flit ; 
His  requiem  sinj;  we  with  shout  and  diu, 
While  the  old  year  goes  out,  and  the  new  comes  In. 
Chorus, — Merrily,  merrily  let  us  all  sing, 

And  make  the  old  telescope  rattle  and  ring  1 


128  PRACTICAL  ASTRONOMY 

The  only  immediate  successor  of  Sir  AV^illiam  Ilerschel  in 
the  construction  of  great  telescopes  was  his  son,  Sir  John  Iler- 
schel. But  the  latter  made  none  to  equal  the  largest  of  his 
fatlier's  in  size,  and  it  is  doubtful  whether  they  exceeded  them 
in  optical  power. 

The  first  decided  advance  on  the  great  telescope  m'rs  the 
celebrated  reflector  of  the  Earl  of  Rosse,*  at  Parsonstown,  Ire- 


F'lill  flfty  years  did  he  laugh  at  the  storm, 
And  the  bhist  could  not  shake  his  majestic  form; 
,  Now  prone  he  lies,  where  he  once  stood  high, 

And  searched  the  deep  heaven  with  his  broad,  bright  eye. 
C/iorMS.— Merrily,  merrily,  etc.,  etc. 

There  are  wonders  no  living  sight  has  seen. 
Which  within  this  hollow  have  pictured  betn ; 
Which  mortal  record  can  never  recall. 
And  are  known  to  Him  only  who  made  them  alL 
Chorus. — Merrily,  merrily,  etc.,  etc. 

Ilcre  watched  our  father  the  wintry  night, 
And  his  giize  has  been  fed  with  preadamite  light. 
His  labors  were  lightened  by  sisterly  love, 
And,  united,  they  strained  their  vision  above. 
Chorus. — Merrily,  merrily,  etc.,  etc. 

He  has  stretched  him  quietly  down,  at  length. 
To  bask  in  the  starlight  his  giant  strength; 
And  Time  shall  here  a  tough  morsel  find 
For  his  steel-devouring  teeth  to  grind. 
C/iocwii.— Merrily,  merrily,  etc.,  etc. 

He  will  grind  it  at  Inst,  as  grind  it  he  must. 
And  its  brass  and  its  iron  sliall  l)e  clay  and  rust ; 
But  scathless  ages  shall  roll  away. 
And  nurture  its  frame,  and  its  form's  decay. 
C/iorM«.— Merrily,  merrily,  etc.,  etc. 

A  new  year  dawns,  and  the  old  year's  past ; 
God  send  it,  a  happy  one  liiie  the  last 
(A  little  more  sun  and  a  little  less  rain 
To  save  us  from  cough  and  rheumatic  pain). 
Chorus. — Merrily,  merrily,  etc.,  etc. 

God  grant  that  its  end  this  group  may  find 

In  love  and  in  harmony  fondly  joined  ! 

And  that  some  of  us,  flfty  years  hence,  once  more 

May  make  the  old  Telescope's  echoes  roar. 

C/iorwa.— Merrily,  merrily,  etc.,  etc. 

*  "William  Parsons,  third  Enrl  of  Itosse,  the  originnl  constructor  of  this  tele- 
scope, died  in  18G7.  The  work  of  the  instrument  is  continued  by  his  son,  the  pres- 
ent  earl. 


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THE  FRINCIPAL  TELESCOPES  OF  MODERN  TIMES.     131 

land.  The  speculum  of  this  telescope  is  six  feet  in  diameter, 
and  ahout  fifty-four  feet  focal  length,  and  was  cast  in  1842. 
One  of  the  great  improvements  made  by  the  Earl  of  Rosse 
was  the  introduction  of  steam  machinery  for  grinding  and 
polishing  the  great  mirror,  an  instrumentality  of  which  Iler- 
scliel  could  not  avail  himself.  The  mounting  of  this  telescope 
is  decidedly  different  from  that  adopted  by  Ilerschel.  The 
telescope  is  placed  between  two  walls  of  masonry,  which  only 
allow  it  to  move  about  10°  on  each  side  of  the  meridian,  and 
it  turns  on  a  pivot  at  the  lower  end  of  the  tube.  It  is  moved 
north  and  south  in  the  meridian  by  an  ingenious  combination 
of  chains,  and  may  thus  be  set  at  the  polar  distance  of  any 
star  which  it  is  required  to  observe.  It  is  then  moved  slowly 
towards  the  west,  so  as  to  follow  the  star,  by  a  long  screw 
driven  by  an  immense  piece  of  clock-work.  It  is  commonly 
used  as  a  Newtonian,  the  observer  looking  into  the  side  of  the 
tube  near  the  upper  end.  To  enable  him  to  reach  the  mouth 
of  the  tube,  various  systems  of  movable  platforms  and  staging 
are  employed.  One  of  the  platforms  is  suspended  south  of 
the  piers ;  it  extends  east  and  west  by  the  distance  between 
the  walls,  and  may  be  raised  by  machinery  so  as  to  be  directly 
under  the  mouth  of  the  telescope  so  long  as  the  altitude  of  the 
latter  is  less  than  45°,  When  the  altitude  is  greater  than  this, 
the  observer  ascends  a  stairway  to  the  top  of  onetof  the  wall:*, 
where  he  mounts  one  of  several  sliding  stages,  by  which  he 
can  be  carried  to  the  mouth  of  the  telescope,  in  any  position 
of  the  latter.  This  instrumenl;  has  been  employed  principal- 
ly in  making  drawings  of  lunar  scenery  and  of  the  planets 
and  nebuliu.  Its  great  light-gathering  power  peculiarly  tits  it 
for  the  latter  object. 

OUicr  Reflecting  Telescopes. — Although  no  other  reflector  ap- 
proaching the  great  one  of  the  Earl  of  Rosse  in  size  has  ever 
been  made,  some  others  are  worthy  of  notice,  on  account  of 
their  perfection  of  figure  and  the  importance  of  the  discov- 
eries made  with  them.  Among  these  the  first  place  is  due  to 
the  great  reflectors  of  Mr.  William  Lassell,  of  England.  This 
gent'  man  made  a  reflector  of  two  feet  aperture  about  the 


132 


PRACTICAL  ASTRONOMY. 


same  time  that  Rosso  constructed  his  immense  six-foot.  The 
perfection  of  figure  of  the  mirror  was  evinced  by  the  discov- 
ery of  two  satellites  of  Uranus,  which  had  been  previously  un- 
known and  unseen,  unless,  as  is  possible,  Ilerschel  and  Struve 
caught  glimpses  of  them  on  a  few  occasions.  He  afterwards 
made  one  of  four  feet  aperture,  which,  in  1863,  he  took  to  the 
island  of  Malta,  where  he  made  a  series  of  observations  on 
satellites  and  nebulae. 


Fio.  41.— Mr.  Lassell's  great  four-foot  refleuior,  us  niouuted  at  Malta. 

In  1870,  a  reflecting  telescope  four  feet  in  diameter,  on  the 
Cassegrainian  plan,  was  made  by  Thomas  Grubb  &  Son,  of 
Dublin,  for  the  Observatory  of  Melbourne,  Australia.  T'liis 
instrimient  is  remarkable,  not  only  for  its  perfection  of  figure, 
but  as  being  probably  the  most  easily  managed  large  reflector 
ever  made. 


t } 


Fio.  42— The  uew  Paris  refleuior. 


rut:  i!,!  •.'"!/■  a    ,/[)■■" -nf':.-  of  s\i'>h'.:u:\    'umis     {A:, 


(A   i^ruM'r,  ..f  New  ^  ock,  \v'in  irti  on'.:  wt'  tw-eiV.'- ciiin   'ticii'i^ 
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Thi-    li-it-M;' iji.'^  ii,'!.-  inMMi  ]>r)t:ci  jrilly  t'm[)lii\  i.il  id   linikii.^-  pi.) 
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THE   PRINCIPAL  TELESCOPES  OF  MODERN  TIMES.     135 

The  only  American  who  has  ever  successfully  undertaken 
the  construction  of  large  reflecting  telescopes  is  Professor  Hen- 
ry Draper,  of  New  York,  who  has  one  of  twenty-eight  inches 
aperture,  the  work  of  his  own  hands.  This  instrument  was 
mounted  about  1872  in  the  owner's  private  observatory  at 
Hastings,  on  the  Hudson.  The  mirror  is  not  of  speculum 
metal,  but  of  silvered  glass,  and  is  almost  perfect  in  figure. 
This  telescope  has  been  principally  employed  in  making  pho- 
tographs of  celestial  objects,  and  can  be  used  either  as  a  New- 
tonian or  a  Cassegrainian. 

An  attempt  has  recently  been  made  at  the  Paris  Observa- 
tory to  construct  a  reflecting  telescope  with  a  mirror  of  sil- 
vered glass,  as  large  as  the  great  specula  of  Lassell  and  the 
Melbourne  Observatory.  The  diameter  of  the  glass  is  120 
centimetres,  a  fraction  of  an  inch  short  of  four  English  feet. 
It  was  figured,  polished,  and  silvered  at  the  Paris  Observa- 
tory by  M.  Martin,  using  the  methods  devised  by  Foucault. 
It  was  mounted  in  1875 ;  but,  unfortunately,  the  proper  meas- 
ures were  not  taken  to  prevent  the  glass  from  bending  under 
its  own  weight,  and  thus  destroying  the  perfection  of  the 
parabolic  figure  which  M.  Martin  had  succeeded  in  obtain- 
ing. It  was  therefore  taken  from  its  tube  to  have  this  defect 
of  mounting  remedied.  The  machinery  for  supporting  and 
moving  this  telescope  being  in  some  respects  peculiar,  we  pre- 
sent a  view  of  it  in  Fig.  42,  on  page  134. 

§  6.   Great  Refracting  Telescopes. 

We  have  already  remarked  that,  in  the  early  days  of  the 
achromatic  telescope,  its  progress  was  hindered  by  the  difli- 
culty  of  making  large  disks  of  flint-glass.  About  the  begin- 
ning of  the  present  century,  Guinand,  a  Swiss  mechanic,  after 
a  long  series  of  experiments,  discovered  a  method  by  which 
he  could  produce  disks  of  flint-glass  of  a  size  before  unheard 
of.  The  celebrated  Fraunhofer  was  then  commencing  busi- 
ness as  an  optician  in  Munich,  and  hearing  of  Guinand's  suc- 
cess induced  him  to  come  to  Munich  and  commence  the  man- 
ufacture of  optical  glass.     Fraunhofer  was  a  physicist  of  a 


136 


VKAVTICAL  ASTRONOMY. 


P'iG.  43. — The  great  Melbourne  reflector.  T,  the  tube  contaiiiiii"^  the  great  mirror  near  itf> 
lower  end.  Y,  the  small  mirror  throwing  the  light  back  to  the  eye-piece,  y.  C  N,  the 
polar  axis.  U,  the  counterpoise  at  the  end  of  the  declination  axis.  Z,  the  clock-work 
which  moves  the  telescope  by  the  jointed  rods  z  ee'E,  and  the  clamp  F. 

liigh  order,  and  made  a  more  careful  and  exhaustive  study  of 
the  optical  qualities  of  glass,  and  the  conditions  for  making 
the  best  telescope,  than  any  one  before  him  liad  ever  attempted. 
With  the  aid  of  the  large  disks  f urnislied  b}'  Guinand,  he  was 
able  to  carry  the  aperture  of  his  telescopes  up  to  ten  inches. 
Dying  in  182G,  his  successors,  Merz  and  Mahler,  of  Munich, 
made  two  telescopes  of  fifteen  inches  aperture,  wliich  were 
tlien  considered  most  extraordinary.     One  of  these  belongs 


GREAT  liEFRACTING   TELESCOPES.  137 

to  the  Pulkowa  Observatory,  iu  Russia ;  and  the  other  was 
purchased  by  a  subscription  of  citizens  of  Boston  for  the  ob- 
servatory of  Harvard  University. 

No  rival  of  the  house  of  Fraunhofer  in  the  construction  ot 
great  refractors  arose  until  he  had  been  dead  thirty  years,  and 
then  it  arose  where  least  expected.  In  1846,  Mr,  Alvan  Clark 
was  a  citizen  of  Canibridgeport,  Massachusetts,  unknown  to 
fame,  who  made  a  modest  livelihood  by  pursuinjjj  the  self- 
taught  art  of  portrait -painting,  and  beguiled  his  leisure  by 
the  construction  of  small  telescopes.  Though  without  the 
advantage  of  a  mathematical  education,  he  had  a  perfect 
knowledge  of  optical  principles  to  just  the  extent  necessary 
to  enable  him  to  make  and  judge  a  telescope.  Having  been 
led  by  accident  to  attempt  the  grinding  of  lenses,  he  soon  pro- 
duced objectives  equal  in  quality  to  any  ever  made,  and,  if 
he  had  been  a  citizen  of  any  other  civilized  country,  would 
have  found  no  difficulty  in  establishing  a  reputation.  But 
he  had  to  struggle  ten  years  with  that  neglect  anu  incre- 
dulity which  is  the  common  lot  of  native  genius  in  this  coun- 
try ;  and,  extraordinary  as  it  may  seem,  it  was  by  a  foreigner 
that  his  name  and  powers  were  first  brought  to  the  notice 
of  tlie  astronomical  world.  Rev.  W.  R.  Dawes,  one  of  the 
leading  amateur  astronomers  of  England,  and  an  acdve  mem- 
ber of  the  Royal  Astronomical  Society,  purchased  an  object- 
glass  from  Mr.  Clark  in  1853.  lie  found  it  so  excellent  that 
in  the  course  of  the  next  two  or  three  years  he  ordered  several 
others,  and,  finally,  an  entire  telescope.  He  also  made  several 
communications  to  the  Astronomical  Society,  giving  lists  of 
difficult  double  stars  detected  by  Mr,  Clark  with  telescopes  of 
his  own  construction,  and  showing  that  Mr.  Clark's  objectives 
were  almost  perfect  in  definition. 

The  result  of  this  \vas  that  the  American  artist  began  to  be 
appreciated  in  his  own  country;  and  in  1860  lie  received  an 
order  from  the  University  of  Mississippi,  of  which  Dr.  F.  A. 
P.  Barnard*  was  then  president,  for  a  refractor  of  eighteen 


Now  President  of  Columbia  College,  New  f '^rk  City. 


138  PRACTICAL  ASTRONOMY. 

inches  aperture,  which  was  three  inches  greater  ciian  the  larg- 
est  that  had  then  been  made.  Before  tlie  glass  was  finished, 
it  was  made  famous  by  the  discovery  of  the  companion  of 
Sirius,  a  success  for  wliich  the  LaLande  medal  was  awarded 
by  the  French  Academy  of  Sciences.  While  this  telescope 
was  in  progress,  the  civil  war  l)roke  out,  and  prevented  the 
party  originally  ordering  it  from  taking  it;  but  it  was  soon 
sold  to  the  Astronomical  Society  of  Chicago,  in  which  city  it 
was  mounted  in  1863.  The  definition  of  this  telescope  is  very 
fine ;  but  the  defects  of  the  dome  in  which  it  is  mounted,  and 
the  want  of  means  to  support  an  astronomer,  have  greatly 
interfered  with  its  efficiency. 

This  instrument  did  not  long  retain  its  supremacy.  The 
firm  of  Thomas  Cooke  <Sz  Sons,  of  York,  England,  in  1870, 
mounted  a  refractor  of  twenty-five  inches  clear  aperture  for 
R.  S.  JXewall,  Esq.,  of  Gateshead,  England,  of  which  the  defi- 
nition is  very  good.  This  instrument  was  intended  by  its 
owner  to  be  transported  to  some  finer  climate  tlian  that  of 
England ;  but  this  project  has  not  been  put  into  execution. 
In  the  summer  of  187'!  it  was  used  by  Mr.  Lockyer,  in  a  study 
of  Coggia's  comet. 

During  the  time  that  these  immense  telescopes  were  being 
made  on  eveiy  hand,  and  after  it  was  proved  that  telescopes  of 
more  than  two  feet  aperture  could  be  made,  the  National  Ob- 
servatory of  the  United  States  had  nothino;  better  than  an  old 
Munich  refractor  of  nine  and  a  half  inches,  such  as  Fraunho- 
fer  used  to  nuike  early  in  the  century.  The  attention  of  Con- 
gress was  so  forcibly  called  to  this  deficiency,  and  to  the  abili- 
ties of  the  firm  of  Alvan  Clark  it  Sons  to  remedy  it,  that,  in 
1870,  a  bill  was  passed  authorizing  the  superintendent  of  the 
observatory  to  contract  for  a  telesco})e  of  the  lai-gest  size  of 
American  manufacture.  The  aperture  agreed  on  was  twenty- 
six  inches,  exceeding  that  of  Mr.  Newall's  telescope  by  only 
one  inch.  It  proved  extremely  ditlicult  to  obtain  disks  of 
rough  glass  even  of  this  size,  and  more  than  a  year  elapsed 
after  Messrs.  Chance  &  Co.  received  the  order  from  Mr.  Clark 
before  they  were  able  to  complete  good  disks  of  the  required 


MAGNIFYING  POWERS  OF  TELESCOPES.  139 

size.  The  glass  arrived  in  December,  1871,  and  work  was  com- 
menced in  January  following.  The  labor  of  polishing  the 
glasses  was  completed  in  October,  1872  ;  the  whole  instrument 
was  completed  in  a  year  more,  and  was  linally  mounted  and 
ready  for  observation  in  November,  1873.  The  figure  of  this 
glass  is  almost  perfect,  its  principal  defect  arising  from  the 
secondary  aberration  which  is  inseparable  from  a  large  re- 
fractor. It  has  been  principally  employed  in  observing  the 
satellites  of  Saturn,  Uranus,  and  Neptune,  with  the  view  of  de- 
termining the  masses  of  these  planets. 

§  7.  The  Magnifijing  Powers  of  the  Two  Classes  of  Telescopes. 

Questions  which  now  very  naturally  arise  are,  Wliich  of  the 
two  classes  of  telescopes  we  have  described  is  the  more  power- 
ful, the  reflector  or  the  refractor?  and  is  there  any  limit  to  the 
magnifying  power  of  either  ?  To  these  questions  it  is  ditlicult 
to  return  a  decided  answer,  because  each  class  has  its  peculiar 
advantages,  and  in  each  class  many  difficulties  lie  in  the  way 
of  obtaining  the  highest  magnifying  power.  The  fact  is,  that 
very  CAaggerated  ideas  of  tiie  magnifying  power  of  great  tele- 
scopes are  entertained  by  the  public.  It  will,  therefore,  be 
instructive  to  state  what  the  circumstances  are  which  pi-event 
these  ideas  from  being  realized,  and  what  the  conditions  are 
on  which  the  seeing  power  of  telescopes  depends. 

We  note,  first,  that  when  we  look  at  a  luminous  point — a  star, 
for  instance — without  a  telescope,  we  see  it  by  the  aid  of  the 
cone  of  light  which  enters  the  pupil  of  the  eye.  The  diameter 
of  the  pupil  being  about  one-fifth  of  an  inch,  as  mnch  light 
from  the  star  as  falls  on  a  circle  of  this  diameter  is  brought  to 
a  focus  on  the  retina,  and  unless  this  quantity  of  light  is  suffi- 
cient to  be  perceptible,  the  star  will  not  be  seen.  Now,  we 
may  liken  the  telescope  to  a  "  Cyclopean  ej'e,"  of  which  the 
object-glass  is  the  pupil,  because,  by  its  aid,  all  the  light  which 
falls  on  the  object-glass  is  brought  to  a  focus  on  the  retina, 
provided  that  a  sufficiently  small  eye-piece  is  used.  Of  course, 
we  must  except  that  portion  of  the  light  which  is  lost  in  pass- 
ing through  the  glasses.     Since  the  quantity  of  light  which 


140  PBACTICAL  ASTRONOMY. 

falls  on  a  surface  is  proportional  to  the  extent  of  the  surface, 
and  therefore  to  the  srpiare  of  its  diameter,  it  follows  that, 
because  a  telescope  of  one -inch  clear  aperture  has  iive  times 
the  diameter  of  the  pupil,  it  will  admit  25  times  the  light;  a 
six-inch  will  admit  900  times  the  light  which  the  pupil  will ; 
and  so  with  any  other  aperture.  A  star  viewed  with  the 
telescope  will,  therefore,  appear  brighter  than  to  the  naked 
eye  in  proportion  to  the  square  of  the  aperture  of  the  in- 
strument. But  the  star  will  not  be  magnified  like  a  planet, 
because  a  point  is  only  a  point,  no  matter  how  often  we  mul- 
tiply it.  It  is  true  that  a  bright  star  in  the  telescope  some- 
times appears  to  have  a  perceptible  disk;  but  this  is  owiug  to 
various  imperfections  of  the  image,  having  their  origin  in  the 
air,  the  insti'ument,  and  the  eye,  all  of  which  have  the  effect  of 
slightly  scattering  a  portion  of  the  light  which  comes  from  the 
star.  Hence,  with  perfect  vision  the  apparent  brilliancy  of  a 
star  will  be  })roportional  to  the  square  of  the  apertui-e  of  the 
telescope.  It  is  said  that  Sir  AVilliam  Ilerschel,  at  a  time  when 
by  accideut  his  telcscoj)e  was  so  pointed  that  Sirius  was  about 
to  enter  its  field  of  view,  was  first  apprised  of  what  was  com- 
ing by  the  appeai'ance  of  a  dawn  like  the  morning.  The  light 
increased  rapidly,  until  the  star  itself  appeared  with  a  dazzling 
splendor  which  remJ-'ded  him  of  the  rising  sun.  Indeed,  in 
any  good  telesco[)e  oi  two  feet  a})erture  or  upwards,  Sirius  is 
an  almost  dazzling  object  to  an  eye  which  has  rested  for  some 
time  in  darkness. 

But  in  order  that  all  the  light  which  falls  on  the  object- 
glass,  or  mirror,  of  a  telescope  may  enter  the  pu|)il  of  the  eye, 
it  is  necessary  that  the  magnifying  power  be  at  hiast  equal  to 
the  ratio  which  the  aperture  of  the  telescope  bears  to  that  of 
the  pupil.  The  latter  is  generally  about  one-fifth  of  an  inch. 
We  nmst,  therefore,  employ  a  magnifying  power  of  at  least 
five  for  every  inch  of  aperture,  or  we  will  not  get  the  full  ad- 
vantage of  our  object-glass.  The  reason  of  this  will  be  appar- 
ent by  studying  Fig.  2t>,  p.  10l>,  from  which  it  will  be  seen  that 
a  pencil  of  parallel  rays  falling  on  the  object-glass,  and  pass- 
ing through  che  eye-piece,  will  be  reduced  in  diameter  in  the 


MAGNIFYING   POWERS  OF  TELESCOPES.  141 

ratio  of  the  focal  distance  of  the  objective  to  that  of  tlie  eye- 
piece, which  is  the  same  as  the  niagnifyiiig  power.  For  in- 
stance, if  to  a  twenty-fonr-inch  telescope  we  attached  an  eye- 
piece so  large  that  the  magnifying  power  was  only  48,  and 
pointed  it  at  a  bright  star,  tlie  "  emergent  pencil  "  of  rays  from 
the  eye-piece  would  be  half  an  inch  in  diameter,  and  the  whole 
of  them  conld  not  possibly  enter  the  pupil.  By  increasing  the 
magnifying  power,  we  would  increase  the  apparent  brilliancy 
of  the  star,  until  we  reached  the  power  120,  after  which  no 
further  increase  of  brilliancy  would  be  possible. 

All  this  supposes  that  we  are  viewing  a  star  or  other  lumi- 
nous point.  If  the  object  has  a  sensible  surface,  like  the  moo"^ 
or  a  large  nebula,  and  we  consider  its  apparent  superficial 
brilliancy,  the  case  will  be  in  part  reversed.  The  object  will 
then  appear  equally  illuminated,  with  all  powers  below  live 
for  each  inch  of  aperture,  but  will  begin  to  grow  darker  when 
we  pass  above  that  limit.  The  reason  of  this  is,  that  as  we 
increase  the  magnifying  power  the  light  is  spread  over  a  larger 
surface  of  the  retina,  and  is  thus  enfeebled.  So  long  as  our 
mngnifying  power  is  below  the  limit,  the  increased  quantity 
of  light  wliich  enters  the  pnpil  by  an  increase  of  magnifying 
power  just  compensates  for  the  greater  surface  over  which  it 
is  spread,  so  that  the  brilliancy  is  constant.  Above  the  limit 
of  live  to  the  inch,  the  surface  over  which  the  light  is  spread, 
or  the  apparent  magnitude  of  the  object,  still  increases  with 
the  magnifying  power,  but  there  is  no  increase  of  light ;  licnce, 
the  object  looks  rainter.  What  may  at  first  sight  seem  para- 
doxical is,  that  the  degree  of  illumination  to  which  we  now 
refer  can  never  be  increased  by  the  use  of  the  telescope,  but, 
at  the  best,  will  be  the  same  as  to  the  naked  eye.  Indeed, 
as  some  light  is  necessarily  lost  in  passing  through  any  tele- 
sco[)e,  the  illumination  is  always  less  with  the  telescope.  With 
the  best  reflectors  of  speculum  metal,  the  illumination  will  be 
reduced  to  one-lialf,  or  less,  if  the  polish  is  not  perfect;  and 
with  refractors  it  will  be  reduced  to  seven  or  eight  tenths.  As 
examples  of  these  conclusions,  the  sky  can  never  bo  made  to 
appear  as  bright  through  a  telescope  as  to  the  naked  eye ;  the 


142  PRACTICAL  ASTRONOMY. 

moon  or  a  large  nebula  will  appear  more  brightly  illuminated 
through  a  refracting  telescope  than  through  a  reflector.  If 
the  object  is  a  very  brilliant  one,  like  the  sun  or  Venus,  the 
loss  of  brilliancy  by  magnifying,  which  we  have  described,  will 
not  cause  any  inconvenience ;  but  the  outer  planets  and  many 
of  the  nebula3  are  so  faintly  illuminated  that  a  magnifying 
power  many  times  exceeding  the  limit  cannot  be  used  with 
advantage. 

Still  another  cause  which  places  a  limit  to  the  power  of 
telescopes  is  diffraction.  When  the  "emergent  pencil"  is 
reduced  below  -i-V  of  ^^  mii\\  in  diameter — that  is,  when  the 
magnifying  power  is  greater  than  50  for  every  inch  of  aper- 
ture of  the  object-glass — the  outlines  of  every  object  observed 
become  confused  and  indistinct,  no  matter  how  bright  the  il- 
lumination or  how  perfect  the  glass  may  be.  The  effect  is  the 
same  as  if  we  looked  through  a  small  pin-hole  in  a  card,  an 
experiment  which  anyone  may  try.  This  effect  is  owing  to 
the  diffraction  of  tlie  light  at  the  edge  of  the  object-glass  or 
mirror,  and  it  increases  so  rapidly  with  the  magnifying  power 
that  when  we  carry  the  latter  above  100  to  the  inch,  the  in- 
crease of  indistinctness  neutralizes  the  increase  of  power.  If, 
then,  we  multi[)ly  the  aperture  of  the  telescope  in  inches  by 
100,  we  shall  have  a  limit  beyond  which  there  is  no  use  in 
magnifying.  Indeed,  it  is  doubtful  if  any  real  advantage  is 
gained  beyond  GO  to  the  inch.  In  a  telescope  of  two  feet  (24 
inches)  aperture  this  limit  would  be  2400.  Such  a  limit  can- 
not be  set  with  entire  exactness ;  but,  even  under  the  most  fa- 
vorable circumstances,  the  advantage  in  attempting  to  surpass 
a  power  of  70  to  the  inch  will  be  very  slight. 

The  foregoing  remarks  ajiply  to  the  most  perfect  telescopes, 
used  under  the  most  favorable  circumstances.  But  the  best 
telescope  has  imperfections  which  would  nearly  always  pre- 
vent the  use  of  the  highest  magnifying  powers  in  astronomical 
observations.  In  the  refracting  telescope  the  principal  defect 
arises  from  the  secondary  aberration  already  explained,  which, 
arising  from  an  inherent  (piality  of  the  glass  itself,  cannot  be 
obviated  by  perfection  of  workmanship.    In  the  case  of  the  re- 


MAGNIFYING  POWERS  OF  TELESCOPES.  143 

fleeter,  the  corresponding  difficulty  is  to  keep  the  mirror  in  per- 
fect figure  in  every  position.  As  the  telescope  is  moved  about, 
the  mirror  is  liable  to  bend,  through  its  own  weight  and  elas- 
ticity, to  such  an  extent  as  greatly  to  injure  or  destroy  the  im- 
age in  the  ^jcus;  and,  though  this  liability  is  greatly  dimin- 
ished by  the  plan  now  adopted,  of  supporting  the  mirror  on  a 
system  of  levers  or  on  an  air-cushion,  it  is  generally  trouble- 
some, owing  to  the  difficulty  of  iceeping  the  apparatus  in  order. 
If  wo  compare  the  refracting  and  reflecting  telescopes  which 
have  hitherto  been  made,  it  is  easy  to  make  a  summary  of 
their  relative  advantages.  If  properly  made  and  attended  to, 
the  refractor  is  easy  to  manage,  convenient  in  use,  and  al- 
ways in  order  for  working  with  its  full  power.  If  its  greatest 
defect,  the  secondary  spectrum,  cannot  be  diminished  by  skill, 
neither  can  it  be  increased  by  the  want  of  skill  on  the  part  of 
the  observer.  So  important  is  this  certainty  of  operation,  that 
far  the  greater  part  of  the  astronomical  observations  of  the 
present  century  have  been  made  with  refractors,  wliich  have 
always  proved  themselves  the  best  working  instruments.  Still, 
the  defects  arising  from  the  secondary  spectrum  are  inlierent 
in  the  latter,  and  increase  with  the  aperture  of  the  glass  to 
such  an  extent  that  no  advantage  can  ever  be  gained  by  carry- 
ing the  diameter  of  the  lenses  beyond  a  limit  wliich  may  be 
somewhere  between  30  and  36  inches.  On  the  otlier  hand, 
when  we  consider  mere  seeing-pow^er,  calculation  at  least  gives 
the  preference  to  the  reflector.  It  is  easy  to  compute  that 
Lord  Ilosse's  "  Leviathan,"  and  the  four-foot  reflectors  of  IMr. 
Lassell  and  of  the  Paris  and  Melbourne  observatories,  must 
collect  from  two  to  four  times  the  liffht  of  the  ffreat  Washing- 
ton  telescope.  But  when,  instead  of  calculation,  we  inquire 
what  difficult  objects  have  actually  been  seen  with  the  two 
classes  of  instruments,  the  result  seems  to  indicate  that  the 
greatest  refractor  is  equal  in  o})tical  ])ower  to  tiie  great  reflect- 
ors. No  known  object  seen  with  the  latter  is  too  faint  to  be 
seen  with  the  former.  Why  this  discrepancy  between  the 
calculated  powers  of  the  great  reflectors  and  their  actual  per- 
formance 'i     The  only  causes  we  can  find  for  it  are  imperfec- 


144  PBACTWAL  ASTRONOMY, 

tions  in  the  figure  ajid  polish  of  tlie  great  mirrors.  The  great 
refractors  are  substantially  perfect  in  their  workmanship;  the 
reflectors  do  not  appear  to  be  perfect,  though  what  the  imper- 
fections may  be,  it  is  impossible  to  say  with  entire  certainty. 
Whether  the  great  telescope  of  the  future  shall  belong  to  the 
one  class  or  the  other  must  depend  upon  whether  the  imper- 
fections of  the  reflecting  mirror  can  be  completely  overcome. 
Mr.  Grubb,  the  maker  of  the  great  Melbourne  telescope,  thinks 
he  has  completely  succeeded  in  this,  so  as  to  insure  a  mirror 
of  six,  seven,  or  even  eight  feet  in  diameter  which  shall  be  as 
perfect  as  an  object-glass.  If  he  is  right  —  and  there  is  no 
mechanician  whose  opinion  is  entitled  to  greater  confidence — 
then  he  has  solved  the  problem  in  favor  of  the  reflector,  so  far 
as  optical  power  is  concerned.  But  so  large  a  telescope  will 
be  so  difficult  to  mani}>ulate,  that  we  must  still  look  to  the  re- 
fractor as  the  working  instrument  of  the  future  as  well  as  of 
the  past;  though,  for  the  discover}^  and  examination  of  very 
faint  objects,  it  may  be  found  that  the  advantage  will  all  be 
on  the  bide  of  the  future  ^reat  reflector. 

The  great  foe  to  astronomical  obserration  is  one  which 
people  seldom  take  into  account,  namely,  the  atmosphere. 
When  we  look  at  a  distant  ol)ject  along  the  surface  of  the 
ground  on  a  hot  summer  day,  we  notice  a  certain  waviness  of 
outline,  accompanied  by  a  slight  trembling.  If  we  look  with 
a  telescope,  we  shall  find  this  waving  and  trembling  magnified 
as  much  as  the  object  is,  so  that  we  can  see  little  better  with 
the  most  powerful  telescope  than  with  the  naked  eye.  The 
cause  of  this  appearance  is  the  mixing  of  the  hot  air  near  the 
crround  with  the  cooler  air  above,  which  causes  an  irresjular 
and  constantly  changing  refraction,  and  tlie  result  is  that  as- 
tronomical observations  requiring  high  magnifying  power  can 
very  rarely  be  advantageously  made  in  the  daytime.  By 
night  the  air  is  not  so  much  disturbed,  yet  there  are  always 
currents  of  air  of  slightly  different  temperatures,  the  crossing 
and  mixing  of  which  produce  the  same  effects  in  a  small  de- 
gree. To  such  currents  is  due  the  twinkling  of  the  stars; 
and  we  may  lay  it  down  as  a  rule,  that  when  a  star  twinkles 


MAGNIFYING  POWERS  OF  TELESCOPES.  145 

the  finest  observation  of  it  cannot  be  made  with  a  telescope  of 
high  power.  Instead  of  presenting  the  appearaiu^e  of  a  bright, 
well-defined  point,  it  will  look  like  a  blaze  of  light  flaring 
about  in  every  direction,  or  like  a  pot  of  molten  boiling  metal ; 
and  the  higher  the  magnifying  power,  the  more  it  will  flare 
and  boil.  The  amount  of  this  atmospheric  disturbance  varies 
greatly  from  night  to  night,  but  it  is  never  entirely  absent. 
If  no  continuous  disturbance  of  the  image  could  be  seen  with 
a  power  of  400,  most  astronomers  would  regard  the  night  as  a 
very  good  one ;  and  nights  on  wliich  a  power  of  more  than 
1000  can  be  advantageously  employed  are  quite  rare,  at  least 
in  this  climate. 

It  has  sometimes  been  said  that  Sir  William  Ilerschel  em- 
ployed a  power  as  high  as  6000  \\\\\i  one  of  his  great  tele- 
scopes, and,  on  the  strength  of  this,  that  the  moon  may  have 
been  brought  within  an  appaient  distance  of  forty  miles.  If 
such  a  power  was  used  on  the  moon,  we  must  suppose,  not 
merely  that  the  moon  was  seen  as  if  at  the  distance  of  forty 
miles,  even  if  Ilerschel  used  his  largest  telescope  —  that  of 
four  feet  aperture — but  that  the  vision  would  be  the  same  as 
if  he  had  looked  through  a  pin-hole  y^  of  an  inch  in  diam- 
eter, and  through  several  yards  of  running  water,  or  aiany 
miles  of  air.  It  is  doubtful  whether  the  moon  has  ever  been 
seen  with  any  telescope  so  well  as  it  could  be  seen  with  the 
naked  eye  at  a  distance  of  500  miles.  If  such  has  been  the 
case,  we  may  be  sure  that  the  magnifying  power  did  not  ex- 
ceed 1000. 

If  seeing  depended  entirely  on  magnifying  power,  we  could 
not  hope  to  gain  much  by  further  improvement  of  the  tele- 
scope, unless  we  should  mount  our  instrument  in  some  place 
where  there  is  less  atmospheric  disturbance  than  in  the  re- 
gions where  observatories  have  hitherto  been  built.  It  is  sup- 
posed that,  on  the  mountains  or  table-lands  in  the  western  and 
south-western  regions  of  North  America,  the  atmosphere  is 
clear  and  steady  in  an  extraordinary  degree ;  and  if  this  sup- 
position is  entirely  correcit,  a  great  gain  to  astronomy  might 
result  from  establishing  an  observatory  in  that  region. 

11 


146  PRACTICAL  ASTRONOMY. 


CIIArXER  II. 

APPLICATION    OF   THE   TELESCOPE   TO   CELESTIAL    MEASUREMENTS. 

§  1.  Circles  of  the  Celestial  Sphere,  and  their  Relations  to  Positions 

on  the  Earth. 

In  the  opening  cliapter  of  tliis  work  it  was  shown  that  all 
the  heavenly  bodies  seem  to  lie  and  move  on  the  surface  of  a 
sphere,  in  the  interior  of  which  the  earth  and  the  observer  are 
placed.  The  operations  of  Practical  Astronomy  consist  large- 
ly in  determining  the  apparent  positions  of  the  heavenly  bod- 
ies on  this  sphere.  These  positions  are  defined  in  a  way  anal- 
ogous to  that  in  which  the  position  of  a  city  or  a  ship  is  de- 
fined on  the  earth,  namely,  by  a  system  of  celestial  latitudes 
and  longitudes.  That  measure  which,  in  the  heavens,  corre- 
sponds most  nearly  to  terrestrial  longitude  is  called  Right  As- 
cension, and  that  which  corresponds  to  terrestrial  latitude  is 
called  Declination. 

In  Fig.  4.5  let  the  globe  be  the  celestial  sphere,  represented 
as  if  viewed  from  the  outside  by  an  observer  situated  towards 
the  cast,  though  we  necessarily  see  the  actual  sphere  from  the 
centre.  Pis  the  north  pole,  ^i?  the  horizon,  Q  the  south  pole 
(invisible  in  northern  latitudes  because  below  the  horizon),  EF 
the  equator,  Z  the  zenith.  The  meridian  lines  radiate  from 
the  north  pole  in  every  direction,  cross  the  equator  at  right 
angles,  and  meet  again  at  the  south  pole,  just  like  meridians 
on  the  earth.  The  meridian  from  which  right  ascensions  are 
counted,  corresponding  in  this  respect  to  the  meridian  of 
Greenwich  on  the  surface  of  the  earth,  is  that  which  passes 
through  the  vernal  equinox,  or  point  of  crossing  of  the  equa- 
tor and  ecliptic.    It  is  called  tiie  first  meridian.    Three  bright 


CIRCLES  OF  THE  CELESTIAL  SPHERE, 


147 


stars  near  wliicli  this  meridian  now  passes  may  be  seen  during 
the  autumn:  they  are  a  Andromeda3  and  y  Pegasi,  on  Maps 
II.  and  v.,  and  /3  Cassiopeiaj,  on  Map  I.  Tlie  riglit  ascension 
of  any  star  on  this  meridian  is  zero,  and  the  right  ascension 
of  any  other  star  is  measured  by  the  angle  which  the  merid- 
ian passing  through  it  makes  with  the  first  meridian,  this  angle 
being  always  counted  towards  the  east.  For  reasons  which 
will  soon  be  explained,  right  ascension  is  generally  reckoned, 
not  in  degrees,  but  in  hours,  minutes,  and  seconds  of  time. 


Fia.  44.— Circles  of  the  celestial  sphere. 


IJ  is  the  ecliptic,  crossing  the  equator  at  its  point  of  inter- 
section with  the  first  meridian,  and  making  an  angle  of  23^'^ 
with  it.  The  declination  of  a  star  is  its  distance  from  the 
celestial  equator,  whether  north  or  south,  exactly  as  latitude 
on  the  earth  is  distance  from  the  earth's  equator.  Thus,  when 
the  right  ascension  and  declination  of  a  heavenly  body  are 
given,  the  astronomer  knows  its  position  in  the  celestial  sphere, 
just  as  we  know  the  position  of  a  city  on  the  earth  when  its 
longitude  and  latitude  are  given. 

It  must  be  observed  that  the  declinations  of  the  heavenlv 


148  PliACTICAL  ASTliONOMY. 

bodies  are,  in  a  certain  sense,  referred  to  the  earth.  In  as- 
tronomy the  equator  is  regarded  as  a  plane  passing  through 
the  centre  of  the  earth,  at  right  angles  to  its  axis,  and  dividing 
it  into  two  hemispheres.  Tlie  line  where  this  plane  intersects 
the  surface  of  the  earth  is  our  terrestrial,  or  geographical,  e(pia- 
tor.  If  an  observer  standing  on  the  geographical  equator  im- 
agines this  plane  running  east  and  west,  and  cutting  into  and 
through  the  earth,  where  he  stands  he  will  have  the  astro- 
nomical equator,  which  differs  from  the  geographical  equator 
only  in  being  the  plane  in  which  the  latter  is  situated.  Now 
imagine  this  plane  continued  in  every  direction  without  limit 
till  it  cuts  the  infinite  celestial  sphere  as  in  Fig.  17,  page  62. 
The  circle  in  which  it  intersects  this  sphere  will  be  the  celes- 
tial equator.  It  will  pass  directly  over  the  head  of  the  ob- 
server at  the  equator. 

There  is  a  general  correspondence  between  latitude  on  the 
earth  and  declination  in  the  heavens,  which  may  be  seen  by 
referring  to  the  same  figure.  Here  the  reader  must  conceive 
of  the  earth  as  a  globe,  ep,  situated  in  the  centre  of  the  celes- 
tial sphere,  EPQS,  which  is  infinitely  larger  than  the  earth. 
The  plane  represented  by  l^Q  is  the  astronomical  equator,  di- 
viding both  the  earth  and  the  imaginary  celestial  sphere  into 
two  equal  hemispheres.  Suppose,  now,  that  the  observer,  in- 
stead of  standing  under  the  equator,  is  standing  \mder  some 
other  parallel,  say  that  of  45°  N.  (Being  in  this  latitude  means 
that  the  plumb-line  where  he  stands  makes  an  angle  of  45° 
with  the  plane  of  the  equator.)  The  point  over  his  head  will 
then  be  in  45°  celestial  declination.  If  we  imagine  a  pencil 
of  infinite  length  rising  vertically  where  the  observer  stands 
so  that  its  point  shall  meet  the  celestial  sphere  in  his  zenith, 
and  if,  as  the  earth  performs  its  diurnal  revolution  on  its  axis, 
we  imagine  this  pencil  to  leave  its  mark  on  the  celestial  si^here, 
this  mark  will  be  the  parallel  of  45°  N.  declination,  or  a  cir- 
cle everywhere  equally  distant  from  the  equator  and  from  the 
pole.  The  same  observer  will  see  the  celestial  pole  at  an  eleva- 
tion equal  to  his  latitude,  that  is,  at  the  angle  45°.  We  have  now 
the  following  rules  for  determining  the  latitude  of  a  place : 


CIRCLES  OF  THE  CELESTIAL  SPHERE.  149 

1.  The  latitude  is  equal  to  the  declination  of  the  observer  s  ztnith. 

2.  It  is  also  equal  to  tJie  altitude  of  tlie  pole  above  Ids  horizon. 
Hence,  if  the  astronomer  at  any  unknown  station  wishes  to 

determine  his  latitude,  he  has  only  to  find  what  parallel  of 
declination  passes  through  his  zenith,  the  latter  being  marked 
by  the  diro(;tion  of  the  plumb-line,  or  by  the  perpendicular  to 
the  surface  of  still  water  or  quicksilver.  If  ho  finds  a  star 
passing  exactly  in  his  zenith,  and  knows  its  declination,  he  has 
his  latitude  at  once,  because  it  is  the  same  as  the  stars  dec- 
lination. Practically,  however,  an  observer  will  never  find  a 
known  star  exactly  in  his  zenith ;  he  must  therefore  find  at 
what  angular  distance  from  the  zenith  a  known  star  passes  his 
meridian,  and  by  adding  or  subtracting  this  distance  from  the 
star's  declination  he  has  his  latitude.  If  he  does  not  know 
the  declination  of  any  star,  he  measures  the  altitudes  above 
the  horizon  at  which  any  star  near  the  pole  passes  the  merid- 
ian, both  above  the  pole  and  under  the  pole.  The  mean  of 
the  two  gives  the  latitude. 

Let  us  now  consider  the  more  complex  problem  of  deter- 
mining longitudes.  If  the  earth  did  not  revolve,  the  observ- 
er's longitude  would  correspond  to  the  right  ascension  of  his 
zenith  in  the  same  fixed  maimer  that  his  latitude  corresponds 
to  its  declination.  But,  owing  to  the  diurnal  motion,  there  is 
no  such  fixed  correspondence.  It  is  therefore  necessary  to 
have  some  means  of  representing  the  constantly  varying  rela- 
tion. 

Wherever  on  the  earth's  surface  an  observer  ma}'  stand,  his 
meridian,  both  terrestrial  and  celestial,  is  represented  astronom- 
ically by  an  imaginary  plane  similar  to  the  plane  of  the  equa- 
tor. This  plane  is  vertical  to  the  observer,  and  passes  through 
the  poles.  It  divides  tlie  earth  into  two  hemispheres,  and  is 
perpendicular  to  the  equator.  In  Fig.  17,  the  celestial  and  ter- 
restrial spheres  are  supposed  to  be  cut  through  by  this  plane ; 
it  cuts  the  earth  when  the  observer  stands  in  a  line  running 
north  and  south  from  pole  to  pole,  and  thus  forms  a  terrestrial 
meridian.  The  same  plane  intersects  the  celestial  sphere  in  a 
great  circle,  which,  rising  above  the  observer's  horizon  in  the 


150  PRACTICAL  ASTRONOMY. 

nortli,  passes  througli  tlie  pole  and  the  zenitli,  and  disappears  at 
the  south  horizon.  Two  observers  north  and  south  of  each 
other  liavc  the  same  meridian  ;  but  in  different  longitudes  they 
have  different  meridians,  whicli,  however,  all  pass  througli  each 
pole. 

In  consequence  of  the  earth's  diurnal  motion,  the  meridian 
of  every  place  is  constantly  moving  among  the  stars  in  such  a 
way  as  to  make  a  complete  revolution  in  23  hours  5G  minutes 
4.09  seconds.  The  reader  will  find  it  more  easy  to  conceive 
of  the  celestial  sphere  as  revolving  from  east  to  west,  the  ter- 
restrial meridian  remaining  at  rest;  the  effect  being  geomet- 
rically the  same  whether  we  conceive  of  the  true  or  the  ap- 
parent motion.  There  are,  then,  two  sets  of  meridians  on 
the  celestial  sphere.  One  set  (that  represented  in  Fig.  45)  is 
fixed  among  the  stars,  and  is  in  constant  apparent  motion 
from  east  to  west  with  the  stars,  while  the  other  set  is  fixed 
by  the  earth,  and  is  apparently  at  rest. 

As  differences  of  latitude  are  measured  bv  angles  in  the 
heavens,  so  differences  of  terrestrial  longitude  are  measured  by 
the  time  it  takes  a  celestial  meridian  to  pass  from  one  terres- 
trial meridian  to  another ;  while  differences  of  right  ascension 
are  measured  by  the  time  it  takes  a  terrestrial  meridian  to 
move  from  one  celestial  meridian  to  another.  Ordinary  solar 
time  would,  however,  be  inconvenient  for  this  measure,  because 
a  revolution  does  not  take  place  in  an  exact  number  of  hours. 
A  different  measure,  known  as  sidereal  time,  is  therefore  in- 
troduced. The  time  required  for  one  revolution  of  the  celes- 
tial meridian  is  divided  into  24  hours,  and  these  hours  are 
subdivided  into  minutes  and  seconds.  Sidereal  noon  at  any 
place  is  the  moment  at  which  the  vernal  equinox  passes  the 
meridian  of  that  place,  and  sidereal  time  is  counted  round 
from  0  hour  to  24  hours,  when  the  equinox  will  have  returned 
to  the  meridian,  and  the  count  is  commenced  over  again. 
Since  right  ascensions  in  the  heavens  are  counted  from  the 
equinox,  when  it  is  sidereal  noon,  or  0  hour,  all  celestial  ob- 
jects on  the  meridian  of  the  place  are  in  0"  of  right  ascension. 
At  1  hour  sidereal  time,  the  meridians  have  moved  15°,  and 


CIRCLES  OF  THE  CELESTIAL  SVUEBE.  151 

objects  now  on  the  meridian  are  in  15°  of  right  ascension. 
Throughout  its  wliole  diurnal  course  the  right  ascension  of  the 
meridian  constantly  increases  at  the  rate  of  15°  per  hour,  so 
tliat  the  right  ascension  is  always  found  by  multiplying  the 
sidereal  time  by  15.  To  avoid  this  constant  multiplication,  it 
is  customary  in  astronomy  to  express  both  right  ascensions  and 
terrestrial  longitudes  by  houi-s.  Thus  the  Pleiades  arc  said  to 
be  in  3  hours  40  minutes  right  ascension,  meaning  that  they  are 
on  the  meridian  of  any  place  at  3  hours  40  minutes  sidereal 
time.  The  longitude  of  the  Washington  Observatory  from 
Greenwich  is  77°  3';  but  in  astronomical  language  the  longi- 
tude is  said  to  be  5  hours  8  minutes  12  seconds,  meaning  that 
it  takes  5  hours  8  minutes  12  seconds  for  any  celestial  merid- 
ian to  pass  from  the  meridian  of  Greenwich  to  that  of  AVash- 
ington.  In  consequence,  when  it  is  0  hour,  sidereal  time  at 
Washington,  it  is  6  hours  8  minutes  12  seconds  sidereal  time 
at  Greenwich. 

About  March  22d  of  every  year,  sidereal  0  hour  occurs  very 
nearly  at  noon.  On  each  successive  day  it  occurs  about  3  min- 
utes 56  seconds  earlier,  which  in  the  course  of  a  year  brings 
it  back  to  noon  again.  Since  the  sidereal  time  gives  the  posi- 
tion of  the  celestial  sphere  relatively  to  the  meridian  of  any 
place,  it  is  convenient  to  know  it  in  order  to  Und  what  stars 
are  on  the  meridian.  Tlie  following  table  shows  the  sidereal 
time  of  mean,  or  ordinary  civil,  noon  at  the  beginning  of  each 
month : 


Urt.  Mill. 

January 18  4') 

Febiiiary 20  47 

March 22  37 

April 0  40 

May 2  .% 

June 4  40 


Hrs.  ATin. 

July G    38 

August 8    40 

September 10   43 

October 12    41 

November 14    43 

December lO   42 


The  sidereal  time  at  any  hour  of  the  year  may  be  found 
from  the  preceding  table  by  the  following  process  within  a 
very  few  minutes:  To  the  number  of  the  preceding  table 
corresponding  to  the  month  add  4  minutes  for  each  day  of 
the  month,  and  the  hour  past  noon.     The  sum  of  these  num- 


152  PRACTICAL  ASTRONOMY. 

bers,  subtracting  24  liours  if  the  sura  exceeds  that  quantity, 
will  give  the  sidereal  time.  As  an  example,  let  it  be  required 
to  5nd  the  sidereal  time  corresponding  to  November  13th  at 
3  A.M.     This  is  15  hours  past  noon.     So  we  liave 

Hrs.    Min. 

November,  from  table 14   4;^ 

i;j  daysx4 0   52 

Past  noon IT)     0 

Sum 30   35 

Subtract 24     0 

Sidereal  time  required 0   35 

The  sidereal  tin.e  obtained  in  this  way  will  seldom  or  never 
be  more  than  live  minutes  in  error  during  the  remainder  of 
this  century.  In  every  observatory  the  principal  clock  runs 
by  sidereal  time,  so  that  by  looking  at  its  face  the  astronomer 
knows  what  stars  are  on  or  near  the  meridian.  Having  the 
sidereal  time,  the  stars  which  are  on  the  meridian  may  be 
found  by  reference  to  the  star  maps,  where  the  right  ascen- 
sions are  shown  on  the  borders  of  the  maps. 

§  2.  Tlie  Meridian  Circle,  and  its  Use. 

As  a  complete  description  of  the  various  sorts  of  instru- 
ments used  in  astronomical  measurements,  and  of  the  modes 
of  using  them,  would  interest  but  a  small  class  of  readers, 
we  shall  confine  ourselves  for  the  present  to  one  which  may 
be  called  the  fundamental  instrument  of  modern  astronomy, 
the  ai>plication  of  which  has  direct  and  immediate  reference 
to  the  circles  of  the  celestial  sphere  described  in  the  preceding 
sect!  n.  iiis  one  is  termed  the  M  ridicui  Circle,  or  'Transit  Cir- 
cle. Its  esbv.  '".'il  parts  are  a  moderate-sized  telescope  balanced 
on  an  axis  passing  through  its  centre,  with  a  system  of  line 
lines  in  the  eye-piece ;  one  or  two  circles  fastened  on  the  axis, 
rcvolviiig  with  the  telescope,  and  having  degrees  and  subdi- 
visions cut  on  their  outer  edges;  and  a  set  of  microscope  mi- 
crometers for  measuring  between  the  lines  so  cut.  It  is  abso- 
lutcly  necessary  that  every  part  of  the  instrument  shall  be  of 
the  most  perfect  'vorknuinship,  and  t'iai;  iliG  niasoiwy  piers  on 


THE  MERIDIAN  CIRCLE,  AND  ITS    USE. 


153 


which  it  is  mounted  shall  he  as  stahle  as  it  is  possihle  to  make 
them. 

There  are  many  differences  of  detail  in  the  construction 
and  mountinsr  of  different  meridian  circles,  but  thev  all  turn 
on  an  east  and  west  horizontal  axis,  and  therefore  the  telescope 
moves  only  in  the  plane  of  the  meridian.     Fig.  45  shows  the 


Fiu.  45.— Tlie  Washiiij;ton  transit  circle. 

construction  of  the  great  circle  in  the  Naval  Observatory, 
Washington.  The  nuirble  piers,  PP,  are  supported  on  a  mass 
of  masonry  nnder  the  floor,  the  bottom  of  which  is  twelve  feet 
below  the  surface  of  the  ground.  The  middle  of  the  telescope 
is  formed  of  a  large  cube,  about  flfteen  inches  on  each  side. 
From  the  east  and  west  side  of  this  cube  extend  the  trunn- 
ions, which  are  so  large  next  the  cube  as  to  be  nearly  conical 
in  shape.  The  outer  ends  terminate  in  finely  ground  steel 
pivots  two  and  a  half  inches  in  diameter,  which  rest  on  brass 
Vs  firmly  fixed  to  heavy  castings  set  into  the  piers  with  by- 


154 


PRACTICAL  ASTRONOMY. 


draulic  cement.  In  order  that  the  delicate  pivots  may  not 
be  worn  by  the  wliole  weight  of  the  instrument  resting  on 
them,  the  counterpoises,  BB,  support  all  the  weight  except  30 
or  40  pounds.  Near  the  ends  of  the  axis  are  the  circles,  seen 
edgewise,  which  are  firmly  screwed  on  the  trunnions,  and  there- 
fore turn  with  the  instrument.  Each  pier  carries  four  arms, 
and  each  of  these  arms  carries  a  microscope,  marked  ??i,  hav- 
ing in  its  focus  the  face  of  the  circle  on  which  the  lines  are 
cut.  These  lines  divide  the  circle  into  360°,  and  each  degree 
into  thirty  spaces  of  two  minutes  each,  so  that  there  are  10,800 
lines  cut  on  the  circle.  They  are  cut  in  a  silver  band,  and  are 
so  fine  as  to  be  invisible  to  the  naked  eye  nnless  the  light  is 
thrown  upon  them  in  a  particular  way.  On  each  side  of  the 
instrument,  in  a  line  with  the  axis,  is  a  lamp  which  throws 
light  into  the  telescope  so  as  to  illuminate  the  field  of  view. 
Reflecting  prisms  inside  of  the  pier  throw  some  of  the  light 
upon  those  points  of  the  cu'cle  which  are  viewed  by  the  mi- 
croscopes, so  as  to  illuminate  the  fine  divisions  on  the  circle. 
Being  thus  limited  in  its  movements,  an  object  can  be  seen 
with  the  telescope  only  when  on,  or  very  near,  the  meridian. 
The  sole  use  of  the  instrument  is  to  observe  the  exact  times 
at  which  stars  cross  the  meridian,  and  their  altitudes  above 
the  horizon,  or  distances  from  the  zenith,  at  the  time  of  cross- 
ing. To  give  precision  to  these  observations,  the  eye-piece  of 
the  instrument  is  supplied  with  a  system  of  fine  black  lines, 

usually  made  of  spider's  web,  as 
shown  in  Fifr.  46.  These  lines 
are  set  in  the  focus,  so  that  the 
image  of  a  star  crossing  the  me- 
ridian passes  over  them.  The 
middle  vertical  sjiidcr  line  marks 
the  meridian  ;  and  to  find  the 
time  of  meridian  transit  of  a  star 
it  is  only  necessary  to  note  ^he 
moment  of  passage  of  its  image 
_    ,„„,,„      ,«,,,■    "r    over  this  line.    I>ut,  to  ffive  ffrcat- 

Fiu.  40.— Spider  lilies  ill  fluid  of  view  of  _   _  '!->*-) 

a  ineriuiau  circle.  er  precision  and  certainty  to  his 


THE  MERIDIAN  CIRCLE,  AND  ITS   USE.  155 

observation,  the  astronomer  generally  notes  the  moments  of 
transit  over  five  or  more  lines,  and  takes  the  average  of  them 
all. 

Formerly  the  astronomer  had  to  find  the  times  of  transit  by 
listening  to  the  beat  of  his  sidereal  clock,  counting  the  sec- 
onds, and  estimating  the  tenths  of  a  second  at  which  the  tran- 
sit over  a  line  tooK  place.  If,  for  instance,  he  should  find  that 
the  star  had  not  reached  the  line  when  the  tick  of  twenty- 
three  seconds  was  heard,  but  crossed  before  the  twenty-fourth 
second  was  ticked,  he  would  know  that  the  time  was  twenty- 
three  seconds  and  some  fraction,  and  would  have  to  estimate 
what  that  fraction  was.  A  skilful  observer  will  generally 
make  this  estimate  within  a  tenth  of  a  second,  and  will  only 
on  rare  occasions  be  in  error  by  as  much  as  two  tenths. 

Shortly  after  the  introduction  of  the  electric-telegraph,  the 
American  astronomers  of  that  day  introduced  a  much  easier 
method  of  determininij;  the  time  of  transit  of  a  star,  bv  means 
of  the  electro-clironofpvjjJi.  As  now  nuide,  this  instrument  con- 
sists of  a  revolving  cylinder,  having  a  sheet  of  paper  wrapped 
around  it,  and  making  one  revolution  per  minute.  A  ])en 
or  other  marker  is  connected  with  a  telegraphic  apparatus  in 
such  a  way  that  whenever  a  signal  is  sent  to  the  pen  it  makes 
a  mark  on  the  moving  paper.  This  pen  moves  lengthwise  of 
the  cylinder  at  the  rate  of  about  an  inch  in  ten  minutes,  so 
that,  in  conse([uen(;e  of  the  turning  of  the  cylinder  on  its  axis, 
the  marks  of  the  pen  will  be  along  a  spiral,  the  folds  of  wliich 
are  one-tenth  of  an  inch  apart.  The  galvanic  circuit  which 
works  the  pen  is  coimectcd  with  the  sidereal  clock,  so  that  the 
latter  causes  tbe  pen  to  make  a  signal  every  second.  The 
same  pen  may  be  worked  by  a  telegraphic  key  in  the  hand 
of  the  observer.  The  latter,  looking  into  his  telescope,  and 
watc^hing  the  a])proa(;h  rf  the  inuige  of  tiie  star  to  each  wire, 
makes  a  signal  at  the  moment  at  which  the  star  crosses.  This 
signal  is  recorded  on  the  chronograph  in  its  proper  place 
among  the  clock  signals,  from  which  it  may  be  distinguished 
by  its  greater  strength.  The  record  is  permanent,  and  the 
sheet  may  be  taken  off  and  read  at  leisure,  the  exact  tenth  of 


156  PRACTICAL  ASTRONOMY. 

a  second  at  which  each  sio-nal  was  made  beiiio;  seen  bv  its 
position  among  the  clock  signals.  Tlie  great  advantages  of 
this  method  are,  that  great  skill  and  practice  are  not  required 
to  make  good  observations,  and  that  the  observer  need  not  see 
either  the  clock  or  his  book,  and  can  make  a  great  number  of 
observations  in  the  course  of  the  evening  which  may  be  read 
off  at  leisure.  In  the  case  of  the  most  skilful  observers  there 
is  no  great  gain  in  accuracy,  for  the  reason  that  they  can  esti- 
mate the  fraction  of  a  second  by  the  eye  and  ear  with  nearly 
the  same  accuracy  that  thcv  can  \A\e  the  siii-ual. 

The  zenith  distance  of  the  star,  from  which  its  declination 
is  determined,  is  observed  by  liaving  in  the  reticule  a  hori- 
zontal spider  line  which  is  n\ade  to  bisect  the  image  of  the 
star  as  it  passes  the  meridian  line.  The  observer  then  goes  to 
the  microscopes,  ascertains  what  lines  cut  on  the  circle  are  un- 
der them,  and  what  number  of  seconds  the  nearest  line  is  from 
the  proper  point  in  the  iield  of  the  microscope.  The  mean  of 
the  results  fi'om  the  four  microscopes  is  called  the  circle-readimj., 
and  can  be  determined  within  two  or  three  tenths  of  a  second 
of  ""c,  or  even  nearer,  if  all  the  apparatus  is  in  the  best  order. 
'  ne  minuteness  of  this  angle  may  be  judged  by  the  circum- 
stance that  the  smallest  round  ol)ject  a  keen  eye  can  see  sub- 
tends an  angle  of  about  forty  seconds. 

AVe  have  described  only  the  leading  operations  necessary  in 
determinations  with  a  meridian  circle.  To  complete  the  de- 
tei'mination  of  the  position  of  a  star  as  accurately  as  a  prac- 
tised observer  can  bisect  it  with  the  spider  line  is  a  much  more 
complicated  matter,  owing  to  the  unavoidable  errors  and  im- 
perfections of  his  instrument.  It  is  impossible  to  set  the  lat- 
ter in  the  meridian  with  mathematical  ])recision,  and,  if  it  M'ere 
done,  it  would  not  remain  so  a  single  day.  When  the  ast'-on- 
omer  comes  to  tenths  of  seconds,  he  has  difficulties  to  contend 
with  at  every  step.  The  effects  of  changes  of  temperature 
and  motions  of  the  solid  earth  on  the  foundations  of  his  in- 
strument are  such  as  to  keep  it  constantly  changing ;  his  clock 
is  so  far  from  going  right  that  he  never  attempts  to  set  it  per- 
fectly right,  but  only  deterniines  its  error  from  his  obscrva- 


DETEEMIXATION  OF  TERRESTRIAL  LONGITUDES.      157 

tions.  Every  observation  must,  therefore,  be  corrected  for  a 
number  of  instrumental  errors  before  the  result  is  accurate, 
an  operation  many  times  more  laborious  than  merely  making 
the  observation. 

§  3.  Determination  of  Terrestrial  Longitudes. 

The  telegraphic  mode  of  recording  observations,  described 
in  the  last  section,  affords  a  method  of  determining  differences 
of  longitude  between  place?  connected  by  telegraph  of  ex- 
traordinary elegance  and  perfection.  We  have  already  shown 
that  the  difference  of  longitude  between  two  points  is  meas- 
ured by  the  time  it  takes  a  star  to  move  from  the  meridian  of 
the  easternmost  point  to  that  of  the  westernmost  point.  We 
have  also  explained  in  the  last  section  how  an  observer  with  a 
meridian  circle  de^3rmines  and  records  the  passage  of  a  star 
over  his  meridian  within  a  tenth  of  a  second.  Since  the  ze- 
nith distance  of  the  star  is  not  required  in  this  observation,  the 
circles  and  microscopes  may  be  dispensed  with,  and  the  instru- 
ment is  tlicn  much  simpler  in  construction,  and  is  termed  a 
Transit  Instrument.  When  the  observer  makes  a  telegraphic 
record  of  the  moment  of  transit  of  a  star  by  striking  a  key  in 
the  manner  described,  it  is  evident  that  the  electro  -  chrono- 
graph on  which  his  taps  are  recorded  may  be  at  any  distance 
to  which  the  electric  current  can  carry  his  signal.  It  may, 
therefore,  be  in  a  distant  city.  There  is  no  difficulty  in  a 
Washington  observer  recording  his  observations  in  Cincinnati. 

On  this  system,  the  mode  of  operation  is  about  as  follows : 
the  Washington  and  Cincinnati  stations  each  has  its  transit  in- 
strument, its  observer,  and  its  chronograph ;  but  the  chrono- 
graphs are  connected  by  telegraph,  so  that  any  signal  made 
by  either  obseror  is  recorded  on  both  chronographs.  As 
the  AVashington  observer  sees  a  star  ]ireviously  agreed  on  pass 
over  the  lines  in  tlie  focus  of  his  instrument,  he  makes  sig- 
nals with  his  telegraphic  key,  wliich  are  recorded  both  on  his 
own  chronograph  and  on  that  of  Cincinnati.  When  tlie  star 
readies  the  meridian  of  the  latter  city,  the  observer  tliere  sig- 
nals the  transit  of  the  star  in  like  manner,  and  the  moment 


158  PRACTICAL  ASTRONOMY. 

of  passage  over  each  lino  in  the  focus  of  his  instrument  is 
recorded,  both  in  Cincinnati  and  Washington.  The  elapsed 
time  is  then  found  by  measuring  off  the  chronograph  sheets. 

The  reason  for  having  all  the  observations  recorded  on  both 
chronographs  is  that  the  results  may  be  corrected  for  the  time 
it  takes  the  electric  current  to  pass  between  the  two  cities, 
which  is  quite  perceptible  at  great  distances.  In  consequence 
of  this  "  wave-time,"  the  Washington  observation  will  be  re- 
corded a  little  too  late  at  Cincinnati,  so  that  the  difference  of 
longitude  on  the  Cincinnati  chronograph  will  be  too  small. 
The  Cincinnati  observation,  which  comes  last,  being  recorded 
a  little  too  late  at  Washington,  the  difference  of  time  on  the 
Washington  chronograph  will  be  a  little  too  great.  The  mean 
of  the  results  on  the  two  chronographs  will  be  the  correct 
longitude,  while  their  difference  will  be  twice  the  time  it  takes 
the  electiic  current  to  pass  between  the  two  cities.  The  re- 
sults thus  obtained  for  the  velocity  of  electricity  are  by  no 
means  accordant,  but  the  larger  number  do  not  differ  very 
greatly  from  8000  miles  per  second. 

A  celestial  meridian  moves  over  the  earth's  surface  at  the 
rate  of  lifteen  degrees  an  hour,  or  a  minute  of  arc  in  four  sec- 
onds of  time.  More  precisely,  this  is  the  rate  of  rotation  of 
the  earth.  The  length  of  a  minute  of  arc  in  longitude  de- 
pends on  the  latitude.  It  is  about  GOOO  feet,  or  a  mile  and  a 
sixth  at  the  equator,  but  diminishes  whether  we  go  north  or 
south,  owing  to  the  approach  of  the  meridians  on  the  globular 
earth,  as  can  be  seen  on  a  glol>e.  In  the  latitude  of  our  Mid- 
dle States  it  is  about  4600  feet,  so  that  the  surface  of  the  earth 
there  moves  over  1150  feet  a  second.  At  the  latitude  of 
Greenwich  it  is  3800  feet,  so  that  the  motion  is  950  feet  per 
second.  Two  skilful  astronomers,  by  makiii;;  a  great  num- 
ber of  observations,  can  determine  the  time  it  takes  the  stars 
to  i)ass  from  one  meridian  to  another  within  one  or  two  hun- 
dredths of  a  seer  '  of  time,  and  can  therefore  make  sure  of 
the  difference  of  longitude  between  two  distant  cities  within 
six  or  eight  yards. 

Of  la(:o  the  telegraphic  method  of  determining  longitudes 


DETERMINATION  OF  TERRESTRIAL  LONGITUDES.      159 

has  been  applied  in  a  way  a  little  different,  though  resting  on 
the  same  principles.  Instead  of  recording  the  transits  of  stars 
on  both  chronographs,  each  observer  determines  the  error  of 
his  clock  by  transits  of  stars  of  which  the  right  ascension  has 
been  carefully  determined.  Each  clock  is  then  connected  with 
both  chronographs  by  means  of  the  telegraphic  lines,  and  made 
to  record  its  'leats  for  the  space  of  a  few  minutes  only.  Thus 
the  difference;  between  the  sidereal  times  at  the  two  stations 
for  the  same  moment  of  absolute  time  can  be  found,  and  this 
difference  is  the  difference  of  lon<i;itude  in  time.  A  few  years 
ago,  when  the  difference  of  longitude  between  points  on  the 
Atlantic  and  Pacific  coasts  was  determined  by  the  Coast 
Surve^^a  clock  in  Cambridge  was  made  to  record  its  beats  on 
a  chronograph  in  San  Francisco,  and  vice  versa.  In  186G,  as 
soon  as  the  Atlantic  cable  had  been  successfully  laid,  Dr.  B.  A. 
Gould  went  to  Europe,  under  the  auspices  of  the  Coast  Su.rvey, 
to  determine  the  difference  of  longitude  between  Europe  and 
America.  Owing  to  the  astronomical  importance  of  this  de- 
termination, it  has  since  been  twice  repeated,  once  under  the 
direction  of  Mr.  Dean,  and,  lastly,  under  that  of  Mr.  Ililgard, 
both  of  the  Survey.  These  three  campaigns  gave  the  follow- 
ing iparate  results  for  the  difference  of  longitude  between 
th  Koyal  Observatory,  Greenwich,  and  the  Naval  Observato- 
ry, Washington : 

lira.  Mill.      Sec, 

Dr.  GouM,  18G7 r>     8     12.11 

Mr.  Dean,  1870 f)     8     12.ir) 

Mr.  Ililgard,  1872 5     8     12.09 

The  extreme  difference,  it  will  be  seen,  is  less  than  a  tenth  of 
a  second,  and  would  probably  have  been  smaller  but  for  the 
numerous  difficulties  attendant  on  a  determination  through  a 
long  ocean  cable,  which  are  much  greater  than  through  a  land 
line. 

The  use  of  the  telegraph  for  the  determination  of  longitude 
is  necessarily  limited,  and  other  methods  must  therefore  gen- 
erally be  used.  The  general  problem  of  determining  a  longi- 
tude, whether  that  of  a  shin  upon  the  ocean  or  of  a  station 


160  PRACTICAL  ASTRONOMY. 

upon  the  land,  depends  on  two  requirements :  (1)  a  knowledge 
of  the  local  time  at  the  station,  and  (2)  a  knowledge  of  the 
corresponding  time  at  Greenwich,  Washington,  or  some  other 
standard  meridian.  The  difference  of  these  two  represents 
the  longitude. 

The  first  determination,  that  of  the  local  time,  is  not  a  diflS- 
cult  problem  when  the  utmost  accuracy  is  not  required.  We 
have  already  shown  how  it  is  determined  with  a  transit  instru- 
ment. But  this  instrument  cannot  be  used  at  all  at  sea,  and 
is  somewhat  heavy  to  carry  and  troublesome  to  set  up  on  the 
land.  For  ships  and  travellers  it  is,  therefore,  much  more  con- 
venient to  nse  a  sextant,  by  which  the  altitude  of  the  sun  or  of 
a  star  above  the  horizon  can  be  measured  with  very  little  time 
or  trouble.  To  obtain  the  time,  the  observation  is  made,  not 
when  the  object  is  on  the  meridian,  but  when  it  is  as  nearly  as 
practicable  east  or  west.  Ilaving  found  the  altitude,  the  calcu- 
lation of  a  spherical  triangle  from  the  data  given  in  the  Nau- 
tical Almanac  at  once  gives  the  local  time,  or  the  error  of  the 
chronometer  on  local  time. 

The  difficult  problem  is  to  determine  the  Greenwich  time. 
So  necessary  to  navigation  is  some  method  of  doing  this,  that 
the  British  Government  long  had  a  standing  offer  of  a  reward 
of  £10,000  to  any  one  who  would  find  a  successful  method 
of  determining  the  longitude  at  sea.  When  the  office  of  As- 
tronomer Koyal  was  established,  which  was  in  1675,  the  duty 
of  the  incumbent  was  declared  to  be  "  to  apply  himself  with 
the  most  exact  care  and  diligence  to  the  rectifying  the  Ta- 
bles of  the  Motions  of  the  Heavens,  and  the  }>laccs  of  the 
Fixed  Stars,  in  order  to  find  out  the  so  much  desired  Longi- 
tude at  Sea  for  the  perfecting  tlie  Art  of  Navigation."  The 
reward  above  referred  to  was  ultimately  divided  between  an 
astronomer,  Tobias  Mayer,  who  made  a  great  improvement  in 
the  tables  of  the  moon,  and  a  watch-maker  who  improved  the 
marine  chronometer. 

The  moon,  making  her  monthly  circuit  of  the  heavens,  may 
be  considered  a  sort  of  standard  clock  from  which  the  astron- 
omer can  learn  the  Greenwich  time,  in  whatever  part  of  the 


DETERMINATION  OY  TERRESTRIAL  LONGITUDES.      IGl 

world  ho  may  find  himself.  This  he  does  by  observing  her  po- 
sitions among  the  stars.  The  Nautical  Almanac  gives  the  pre- 
dicted distance  of  the  moon  from  certain  other  bodies — sun, 
planets  or  bright  stars — for  every  three  hours  of  Greenwich 
time;  and  if  the  abtronomer  or  navigator  measures  this  dis- 
tance with  a  sextant,  he  has  the  means  of  finding  at  what 
Greenwich  time  the  distance  was  equal  to  that  measured.  Un- 
fortunately, however,  this  operation  is  n)uch  like  that  of  deter- 
mining the  time  from  a  clock  which  has  nothing  but  an  hour- 
hand.  The  moon  moves  among  the  stars  only  about  13°  in 
a  day,  and  her  own  diameter  in  an  hour.  If  the  observer  wants 
his  Greenwich  time  within  half  a  minute,  he  must  determine 
the  position  of  the  moon  within  the  hundred  and  twentieth  of 
her  diameter.  This  is  about  as  near  as  an  ordinary  ot)servcr 
at  sea  can  come  with  a  sextant ;  and  yet  the  error  would  be  7^ 
miles  of  longitude.  Even  this  degree  of  exactness  can  be  ob- 
tained only  by  having  the  moon's  place  relatively  to  the  stars 
predicted  v;ith  great  accuracy ;  and  here  we  meet  with  one  of 
the  most  complex  problems  of  astronomy,  the  efforts  to  solve 
which  have  already  been  mentioned. 

In  addition  to  the  uncertainty  of  which  we  have  spoken, 
this  method  is  open  to  the  objection  of  being  difficult,  owing 
to  the  long  calculation  necessary  to  free  the  measured  distance 
from  the  effects  of  the  refraction  of  both  bodies  by  the  atmos- 
phere, and  of  the  parallax  of  the  moon.  On  ordinary  voyages 
navigators  prefer  to  trust  to  their  chronometers.  The  error  of 
the  chronometer  on  Greenwich  time  and  its  daily  rate  are 
determined  at  ports  of  which  the  longitude  is  known,  and  the 
navigator  can  then  calculate  this  error  on  the  supposition  that 
the  chronometer  gains  or  loses  the  same  amount  every  day. 
On  voyages  between  Europe  and  America  a  good  chronome- 
ter will  not  generally  deviate  more  than  ten  or  fifteen  seconds 
from  its  calculated  rate,  so  that  it  answers  all  the  purposes  of 
navigation. 

Still  another  observation  by  which  Greenwich  time  may  be 
obtained  to  a  minute  in  any  part  of  the  world  is  that  of  the 
eclipses  of  Jupiter's  first  satellite.     The  Greenwich  or  Wasli- 

12 


162  rHACTWAL  ASTRONOMY. 

ington  times  at  Avhicli  tlie  eclipses  are  to  occur  arc  given  in 
the  NauttcaL  Almanac,  so  tiiat  if  the  traveller  can  succeed  in 
observing  one,  he  has  his  Greenwich  time  at  once,  without  any 
calculation  whatever.  But  the  error  of  liis  observation  nuiy 
be  half  a  minute,  or  even  an  entire  minute,  so  that  this  meth- 
od is  not  at  all  accurate. 

Where  an  astronomer  can  fit  up  a  portable  observatory,  the 
observation  of  the  uioon  affords  him  a  nuich  more  accurate 
longitude  than  it  does  the  navigator,  because  he  can  use  better 
instruments.  If  he  has  a  transit  instrument,  he  determines 
from  observation  the  right  ascension  of  the  moon's  limb  as 
she  passes  his  meridian,  and  then,  referring  to  the  Nautical 
Ahnanac,  he  finds  at  what  Greenwich  time  the  limb  had  this 
right  ascension.  A  single  transit  would,  if  the  moon's  place 
were  correctly  predicted,  give  a  longitude  correct  within  six 
or  eight  seconds  of  time.  It  is  found,  however,  that,  owing  to 
the  errors  of  the  moon's  tables,  it  is  necessary  for  the  astron- 
omer to  wait  for  corresponding  observations  of  the  moon  at 
some  standard  observatory  before  he  can  be  sure  of  this  de- 
gree of  accuracy. 

§  4.  Mean,  or  Clock,  Time. 

We  have  hitherto  described  only  sidereal  time,  which  corre- 
sponds to  the  diurnal  revolution  of  the  starry  sphere,  or,  more 
exactly  yet,  of  the  vernal  equinox.  Such  a  measure  of  time 
would  not  answer  the  purposes  of  civil  life,  and  even  in  astron- 
omy its  use  is  generally  confined  to  the  determination  of  right 
ascensions.  Solar  time,  regulated  by  the  diurnal  motion  of  the 
sun,  is  ahnost  universally  used  in  astronomical  observations  as 
well  as  in  civil  life.  Formerly,  solar  time  was  made  to  con- 
form absolutely  to  the  motion  of  the  sun  ;  that  is,  it  was  noon 
when  the  sun  was  on  the  meridian,  and  the  hours  were  those 
that  would  be  given  by  a  sundial.  If  the  interval  between 
two  consecutive  transits  of  the  sun  were  always  the  same, 
this  measure  would  have  been  adhered  to.  But  there  are  two 
sources  of  variation  in  the  motion  of  the  sun  in  riijht  ascen- 
sion,  the  effect  of  which  is  to  make  these  intervals  unequal : 


MEAN,  OR  CLOCK,  TIME.  163 

1.  The  eccentricity  of  the  earth's  orbit.  In  consequence 
of  this,  as  already  exphiined,  the  angiihir  motion  of  the  earth 
round  the  sun  is  more  rapid  in  December,  when  the  earth  is 
nearest  the  sun,  than  in  June,  when  it  is  farthest.  The  aver- 
age, or  mean,  motion  is  such  that  the  sun  is  3  minutes  56  sec- 
onds longer  in  returning  to  the  meridian  than  a  star  is.  But, 
owing  to  the  eccentricity,  this  motion  is  actually  one-thirtieth 
greater  in  December,  and  the  same  amount  less  in  June ;  so 
that  it  varies  from  3  minutes  48  seconds  to  4  minutes  4  sec- 
onds. 

2.  The  principal  source  of  the  inequality  referred  to  is  the 
obliquity  of  the  ecliptic.  When  the  sun  is  near  the  equinoxes, 
his  motion  among  the  stars  is  oblique  to  the  direction  of  the 
diurnal  motion;  w'hile  the  latter  motion  is  directly  to  the 
west,  the  former  is  23^°  north  or  south  of  east.  If,  then,  sun 
and  star  cross  the  meridian  together  one  day  near  the  equinox, 
he  will  not  be  3  minutes  56  seconds  later  than  the  star  in 
crossing  the  next  day,  but  about  one -twelfth  less,  or  20  sec- 
onds. Therefore,  at  the  times  of  the  equinoxes,  the  solar  days 
are  about  20  seconds  shorter  than  the  average.  At  the  sol- 
stices, the  opposite  effect  is  produced.  The  sun,  being  23^° 
nearer  the  pole  than  before,  the  diurnal  motion  is  slower,  and 
it  takes  the  sun  20  seconds  longer  than  the  regular  interval  of 
3  minutes  56  seconds  for  that  motion  to  carry  the  sun  over 
the  space  which  separates  him  from  the  star  which  culminat- 
ed with  him  the  day  before.  The  days  are  then  20  seconds 
longer  than  the  average,  from  this  cause. 

So  long  as  clocks  could  not  be  made  to  keep  time  within 
20  seconds  a  day,  these  variations  in  the  course  of  the  sun 
were  not  found  to  cause  any  serious  inconvenience.  But 
when  clocks  began  to  keep  time  better  than  the  sim,  it  be- 
came necessary  either  to  keep  putting  them  ahead  when  the 
sun  went  too  fast,  and  behind  when  he  went  too  slow,  or  to 
give  up  the  attempt  to  make  them  correspond.  The  latter 
course  is  now  universally  adopted,  where  accurate  time  is  re- 
quired ;  the  standard  sun  for  time  being,  not  the  real  sun,  but 
a  "  mean  sun,"  which  is  sometimes  ahead  of  the  real  one.  and 


164  PRACTICAL  ASTRONOMY. 

sometimes  beliind  it.  Tlie  irregular  time  deiiending  on  the 
motion  of  the  true  sun,  or  that  given  by  a  sundial,  is  called 
Aiiparent  Time,  while  that  given  by  the  mean  sun,  or  by  a 
clock  going  at  a  uniform  rate,  is  called  Mean  Time.  The  two 
measures  coincide  four  times  in  a  year ;  during  two  interme- 
diate seasons  the  mean  time  is  ahead,  and  during  two  it  is 
behind.  The  following  are  the  dates  of  coincidence,  and  of 
maximum  deviation,  which  vary  but  slightly  from  year  to 
year : 

February  10th True  sun  !'>  minutes  slow. 

Ai)ril  lath "  "  correct. 

May  14th "  "  4  minutes  fast. 

June  14th "  '•  correct. 

July  25th "  "  G  minutes  slow. 

August  ;51st "  "  '     rect. 

November  2d "  "  Id  minutes  fast. 

December  24th "  "  correct. 

When  the  sun  is  slow,  it  passes  the  meridian  after  mean  noon, 
and  the  clock  is  faster  than  the  sundial,  and  vice  versa.  Tiiese 
wide  deviations  are  the  result  of  the  gradual  accumulations  of 
the  deviations  of  a  few  seconds  from  day  to  day,  the  cause  of 
which  has  just  been  explained.  Thus,  during  the  interval  be- 
tween November  2d  and  February  12th,  the  sun  is  constantly 
falling  behind  the  clock  at  an  average  rate  of  18  or  19  seconds 
a  day,  which,  continued  through  100  days,  brings  it  from  16 
minutes  fast  to  15  minutes  slow. 

This  difference  between  the  real  and  the  mean  sun  is  called 
the  Equation  of  Time.  One  of  its  effects,  which  is  frequently 
misunderstood,  is  that  the  interval  from  sunrise  until  noon,  as 
given  in  the  almanacs,  is  not  the  same  as  that  between  noon 
and  sunset.  This  often  leads  to  the  inquiry  whether  the  fore- 
noons can  be  longer  or  shorter  than  the  afternoons.  If  by 
"  noon  "  we  meant  the  passage  of  the  real  sun  across  the  me- 
ridian, they  could  not ;  but  the  noon  of  our  clocks  being  some- 
times 15  mimitcs  before  or  after  noon  by  the  sun,  the  former 
may  be  half  an  hour  nearer  to  sunrise  than  to  sunset,  or  vice 
versa. 


PAIiALLAX  IN  GENEIiAL.  165 


CHAPTER  III. 

MEASURING   DISTANCES   IN   THE   HEAVENS. 

§  1.  Parallax  in  General. 

The  determination  of  the  distances  of  the  heavenly  bodies 
from  us  is  a  much  more  complex  problem  than  merely  deter- 
mining their  apparent  positions  on  the  celestial  sphere.  The 
latter  depend  entirely  on  the  direction  of  the  bodies  from  the 
observer ;  and  two  bodies  which  lie  in  the  same  direction  will 
seem  to  occupy  the  same  position,  no  matter  how  much  farther 
one  may  be  than  the  other.  Notwithstanding  the  enormous 
differences  between  the  distances  of  different  heavenly  bodies, 
there  is  no  wav  of  telling  even  which  is  farthest  and  which 
nearest  by  mere  inspection,  mucli  less  can  the  absolute  dis- 
tance be  determined  in  this  way. 

The  distances  of  the  heavenly  bodies  are  ge orally  deter- 
mined from  their  Parallax.  Parallax  may  be  defined,  in  the 
most  general  way,  as  the  difference  heliccen  the 
directions  of  a  bod//  as  see) i  from  two  different 
points.  Other  conditions  being  equal,  the 
more  distant  the  bodv,  the  less  this  differ- 
ence,  or  the  less  the  parallax.  To  show,  in 
the  most  elementary  way,  how  difference  of 
direction  depends  on  distan.\  suppose  an 
observer  at  0  to  see  two  lights,  A  and  B,  at 
night,     lie  cannot  tell  by  inero   inspection    fo  A 

which  is  the  more  distant.     But  suppose  he  Fio.47.-Diagrnniiiiti?- 
walks  over  to  the  point  P.     Poth  lights  will       trating  paraiiux. 
then  seem  to  change  their  direction,  moving  in  the  direction 
opposite  to  that  in  which  he  goes.    Put  tlu;  light  vl  will  change 
more  than  the  light  B,  for,  being  to  the  right  of  B  when  the 


%' 

k\i\ 

/»^,i.  A 

-^\ 

1   f    ^   \ 

1 
1 

1 

A 

\\ 

// 

\ 

\ 

/ 

ii 

16G  PRACTICAL  ASTRONOMY. 

observer  was  at  0,  it  is  now  to  the  left  of  it.     Tlie  observer 
can  then  say  with  entire  certainty  that  A  is  nearer  tlian  B. 

As  a  steamship  crosses  'he  ocean,  near  objects  at  rest 
change  their  direction  rapidly,  and  soon  flit  by,  while  more 
distant  ones  change  very  slowly.  The  stars  are  not  seen  to 
change  at  all.  If,  however,  the  moon  did  not  move,  the  prs- 
senger  would  see  her  to  have  changed  her  apparent  positi^  n 
about  one  and  a  half  times  her  diameter  in  consequence  of 
the  journey.  If,  when  the  moon  is  near  the  meridian,  an  ob- 
server could  in  a  moi.uent  jump  from  New  York  to  Liverpool, 
kee])ing  his  eye  fixed  upon  her,  he  would  see  her  apparently 
jump  in  the  opposite  direction  about  this  amount. 

Astronomically,  the  direction  of  an  object  from  an  observer 
is  determined  by  its  position  on  the  celestial  sphere;  that  is, 
by  its  right  ascension  and  declination.  In  consecpience  of 
parallax,  the  declination  of  a  body  is  not  the  same  when  seen 
from  different  parts  of  the  earth.  As  the  moon  passes  the 
meridian  of  the  Cape  of  Good  Hope,  her  measured  declina- 
tion may  be  a  degree  or  more  farther  north  than  it  is  when 
she  passes  the  meridian  of  Greenwich.  The  determination  of 
the  parallax  of  the  moon  was  one  of  the  objects  of  the  British 
Govei'nment  in  establisliing  an  observatory  at  the  Cape,  and 
80  well  has  this  object  been  attained  that  the  best  determina- 
tions of  the  parallax  have  been  made  by  comparing  the  Green- 
wich and  Cape  observations  of  the  moon's  declination. 

The  determination  of  the  distance  of  a  celestial  object  from 
the  parallax  depends  on  the  solution  of  a  triangle.  If,  in  Fig. 
48,  we  suppose  the  circle  to  represent  the  earth,  and  imagine 

an  observer  at  A  to  view  a  celes- 
tial object,  J/,  he  will  see  it  pro- 
jected on  th'^  infinite  celcbtial 
sphere  in  the  direction  yl J/ con- 
tinued. Another  observer  at  A' 
will  see  it  in  the  direction  A'M. 
The  difference  of  these  directions 
is  the  angU;  at  }f.  Knowing  all 
Fio.  48.— Dinfe'iura  illustrating  pariiiiai.  the   aiiglcs    of  the   quadrilateral 


PAEALLAX  IN  GENERAL. 


167 


ACA'M,  and  the  Ipiigth  cf  the  earth's  radhis,  CA,  the  dis- 
tance of  the  object  from  the  tln-ee  points,  A,  A',  and  C,  can 
be  found  by  solving  a  s'uiple  problem  of  trigonometry. 

The  term  parallax  is  frequently  used  in  a  more  limited 
sense  than  that  in  which  we  have  just  defined  and  elucidated 
it.  Instead  of  the  difference  of  directions  of  a  celestial  body 
seen  from  any  two  points,  the  astronomer  generally  means  the 
difference  between  tlie  direction 
of  the  body  as  it  would  appear 
from  the  centre  of  the  earth,  ard 
the  direction  seen  by  an  obsei'ver 
at  the  surface.  Thus,  in  Fig.  49, 
an  observer  at  the  centre  of  the 
earth,  C,  would  see  the  object  Al' 
in  the  direction  CJ/',  while  one 
on  the  surface  at  P  will  see  it  in 
the  direction  PM'.  The  differ- 
ence   of   these    directions    is    the  Fiu.  49.— Vmiation  of  parallax  with  the 

angle    PAfC.     If  the   observer  ""'""^''- 

should  be  at  the  point  where  the  line  M'C  intersects  the  sur- 
face of  the  earth,  there  would  be  no  ])arallax :  in  this  case, 
the  object  would  be  in  his  geocentric  zenith.  If,  on  the  other 
hand,  the  observer  has  the  object  in  liis  liorizon,  so  that  the 
line  P}["  is  tangent  to  the  surface  of  the  earth,  the  angle 
CM'Ph  called  the  horizontal  parallax.  The  liorizontal paral- 
lax is  equal  to  the  angle  which  the  radius  of  (he  earth  subtends  as 
seen  from  the  object.  When  we  say  that  the  horizontal  parallax 
of  the  moon  is  57",  and  that  of  the  sun  8".85,  it  is  tlie  same 
thing  as  saying  that  the  diameter  of  tlie  earth  subtends  twice 
tbose  angles  as  seen  from  the  moon  and  sun  respectively. 

Owing  to  the  ellipticity  of  the  earth,  all  its  diameters  will 
not  subtend  the  same  angle;  the  polar  diameter  being  the 
shortest  of  all,  and  the  equatorial  the  longest.  The  equatorial 
diameter  is,  therefore,  adopted  by  astronomers  as  the  standard 
for  parallax.  The  corresponding  parallax,  that  is,  the  equato- 
rial radius  of  the  earth  as  seen  from  a  celestial  body,  is  called 
the  E(iuatorial  Horizontal  Parallax  of  that  body. 


108  PRACTICAL  ASTEOXOMY. 

To  measure  directly  the  distance  of  the  moon  or  any  other 
heavenly  body,  the  line  PO  must  be  replaced  by  the  line  join- 
ing the  positions  of  the  two  observers,  called  the  base-line. 
Knowing  the  length  and  direction  of  this  base-line,  and  the 
difference  of  directions,  or  parallax,  the  distance  is  at  once  ob- 
tained. If  the  absolute  len<i;th  of  the  base-line  should  not  be 
known,  the  astronomer  could  still  determine  the  proportion 
of  the  distance  of  the  object  to  the  base-line,  leaving  the  iinal 
determination  of  the  absolute  distances  to  be  made  wlicn  the 
base-line  could  be  measured. 

It  is  not  always  necessary  for  two  observers  actually  to  sta- 
tion themselves  in  two  distant  par^i  of  the  earth  to  determine 
a  parallax.  If  the  observer  himself  could  move  along  the 
base-line,  and  keep  up  a  series  of  observations  on  the  object,  to 
see  how  it  seemed  to  move  in  the  opposite  direction,  he  would 
still  be  able  to  determine  its  distance.  Now,  every  observer  is 
actually  carried  along  by  two  such  motions,  because  he  is  on 
the  moving  earth.  He  is  carried  round  the  sun  every  year, 
and  round  the  axis  of  the  earth  every  day.  We  have  already 
shown  how,  in  consequence  of  the  first  motion,  all  the  planets 
seem  to  describe  a  series  of  epicycles.  This  apparent  motion 
is  an  effect  of  parallax,  and  by  means  of  it  the  proportions  of 
the  solar  system  can  be  determined  with  extreme  accuracy. 
The  base-line  is  the  diameter  of  the  earth's  orbit.  But  the 
parallax  in  question  does  not  help  us  to  determine  this  base- 
line. To  find  it,  we  must  first  know  the  distance  of  the  earth 
from  the  sun,  and  here  we  have  no  base-line  but  the  diameter 
of  the  earth  itself.  Nor  can  the  annual  motion  of  the  earth 
round  the  sun  enable  us  to  determine  the  distance  of  the 
moon,  because  tlie  latter  is  carried  round  by  the  same  motion. 

The  result  of  the  daily  revolution  of  tlie  observer  round  the 
earth's  axis  is,  that  the  apparent  movement  of  the  planet  along 
its  course  is  not  perfectly  uniform  :  when  the  observer  is  east, 
the  planet  is  a  little  to  the  west,  and  vice  versa.  By  observing 
the  small  inequalities  in  the  motion  of  the  planet  correspond- 
ing to  the  rotation  of  the  earth  on  its  axis,  we  have  the  means 
of  observing  its  distance  with  the  earth's  diameter  as  a  base- 


PARALLAX  IX  GENERAL.  1G9 

line,  and  this  diameter  is  well  known.  Unfortunately,  how- 
ever, the  earth  is  so  small  compared  with  the  distances  of  the 
planets,  that  the  parallax  in  question  almost  eludes  measure- 
ment, except  in  the  case  of  those  planets  which  are  nearest 
the  earth,  and  even  then  it  is  so  minute  that  its  accurate  de- 
termination is  one  of  the  most  difficult  problems  of  modern 
astronomy. 

The  principal  difficulty  in  determining  a  parallax  from  the 
revolutioi.  of  the  observer  around  the  earth's  axis  is  that  the 
observations  are  not  to  be  made  in  the  meridian,  but  when  the 
planet  '"-  near  the  horizon  in  the  east  and  west.  Hence  the 
most  accurate  and  convenient  instrument  of  all,  the  meridian 
Lirr\c,  oimnot  be  used,  and  recourse  nuist  be  had  to  methods 
of  obser\ation  subject  to  many  sources  of  error. 

In  measuring  very  minute  parallaxes,  it  may  be  doubtful 
whether  the  position  of  the  body  on  the  celestial  sphere  can 
be  determined  with  the  necessary  accuracy.  In  this  case  re- 
sort is  sometimes  had  to  relative  parallax.  By  this  is  meant 
the  difference  between  the  parallaxes  of  two  bodies  lying  near- 
ly in  the  same  direction.  The  most  notable  example  of  this 
is  aff  >rdrd  by  a  transit  of  Venus  over  the  face  of  the  sun. 
To  determine  the  absolute  direction  of  Venus  when  nearest 
the  earth  with  the  accuracy  required  in  measurements  of  par- 
allax has  not  hitherto  been  found  practicable,  because  the  ob- 
servation must  be  made  in  the  daytime,  when  the  atmosphere 
is  much  disturbed  by  the  rays  of  the  sun,  and  also  because 
only  a  small  part  of  the  planet  can  then  be  seen.  But  if  the 
planet  is  actually  between  us  and  the  sun,  so  as  to  be  seen  ]>ro- 
jected  on  tho  sun's  face,  the  apparent  distance  of  the  planet 
from  the  centre  or  from  the  limb  of  the  sun  may  be  found 
with  considerable  accuracy.  Moreover,  this  distance  will  be 
different  as  seen  from  different  parts  of  the  earth's  surface  at 
the  same  moment,  owing  to  the  effect  of  parallax ;  that  is,  dif- 
ferent observers  will  see  Venus  projected  on  different  parts  of 
the  sun's  face.  But  the  change  thus  observed  will  be  only 
that  due  to  the  difference  of  the  parallaxes  of  the  two  l)odies; 
while  both  change  their  directions,  that  nearest  the  observer 


170  PRACTICAL  ASTRONOMY. 

changes  the  more,  and  thus  seems  to  move  past  tlie  other,  ex- 
actly as  in  tlie  diagram  of  the  liglits. 

It  may  be  asked  how  the  parallax  of  the  sun  can  be  found 
from  observations  of  the  transit  of  Venus,  if  such  observations 
show  only  the  difference  between  the  parallax  of  Venus  and 
that  of  the  sun.  We  reply  that  the  ratio  of  the  parallaxes  of 
the  two  bodies  is  known  with  great  precision  from  the  propor- 
tions of  the  system.  We  have  already  shown  that  these  pro- 
portions are  known  with  great  accuracy  from  the  third  law  of 
Kepler,  and  from  the  annual  parallax  produced  by  the  revolu- 
tion of  the  earth  round  the  sun.  It  is  thus  known  that  at  the 
time  of  the  transit  of  Venus,  in  1874,  the  sun  was  nearly  four 
times  the  distance  of  Venus,  or,  more  exactly,  that  he  was 
3.783  times  as  far  as  that  planet.  Consequently,  the  parallax 
of  Venus  was  then  3.783  times  that  of  the  sun.  The  differ- 
ence of  the  parallaxes,  that  is,  the  relative  parallax,  must  then 
have  been  2.783  times  the  sun's  parallax.  Consequently,  we 
have  only  to  divide  the  relative  parallax  found  from  the  ob- 
servations by  2.783  to  have  the  parallax  of  the  sun  itself. 

Still  another  parallax,  seldom  applied  except  to  the  fixed 
stars,  is  the  Annual  Parallax.  This  is  the  parallax  already  ex- 
plained as  due  to  the  annual  revolution  of  the  earth  in  its  or- 
bit. It  is  equal  to  the  angle  subtended  by  the  line  joining  the 
earth  and  sun,  as  seen  from  the  star  or  other  body.  When  we 
say  that  the  annual  parallax  of  a  star  is  one  second  of  arc,  it  is 
the  same  thing  as  saying  that  at  the  star  the  line  joining  the 
earth  and  sun  would  subtend  an  apparent  angle  of  one  sec- 
ond, or  that  the  diameter  of  the  earth's  orbit  would  appear  un- 
der an  angle  of  two  seconds. 

It  will  be  seen  that  the  measurement  of  the  heavens  involves 
two  separate  operations.  The  one  consists  in  the  determina- 
tion of  the  distance  between  the  earth  and  the  sun,  which  is 
made  to  depend  on  the  solar  parallax,  or  the  angle  which  the 
semidiameter  of  the  earth  subtends  as  seen  from  the  sun,  and 
which  is  the  unit  of  distance  in  celestial  measurements.  The 
other  consists  in  the  determination  of  the  distances  of  the  stars 
and  planets  in  terms  of  tnis  unit,  which  gives  what  we  may 


MEASURES  OF  THE  DISTANCE  OF  THE  SCIX.  171 

call  the  proportions  of  the  universe.  Knowing  this  proportion, 
we  can  determine  all  the  distances  of  the  universe  when  the 
length  of  our  unit  or  the  distance  of  the  sun  is  known,  but  not 
before.  The  determination  of  this  distance  is,  therefore,  one 
of  the  capital  problems  of  astronomy,  as  well  as  one  of  the  most 
difficult,  to  the  solution  of  which  both  ancient  and  modern  as- 
tronomers have  devoted  many  efforts. 

§  2.  Measures  of  the  Distance  of  the  Sun. 

We  have  already  shown,  in  describing  the  phases  of  the 
moon,  how  Aristarchus  attempted  to  determine  the  distance 
of  the  sun  by  measuring  the  angle  between  the  sun  and  the 
moon,  when  the  latter  appeared  half  ilhnninated.  From  this 
measure,  tlie  sun  was  supposed  to  be  twenty  times  as  far  as 
the  moon ;  a  result  which  arose  solely  from  the  accidental  er- 
rors of  the  observations. 

Another  method  of  attacking  the  problem  was  applied  by 
Ptolemy,  but  is  probably  due  to  Ilipparchus.  It  rests  on  a 
very  ingenious  geometrical  construction  founded  on  the  prin- 
ciple that  the  more  distant  the  sun,  the  narrower  will  be  the 
shadow  of  the  earth  at  the  distance  of  the  moon.  The  actual 
diameter  was  determined  from  an  ingenious  combination  of 
two  partial  eclipses  of  the  moon,  in  one  of  which  half  of  the 
moon  was  south  of  the  limit  of  the  shadow,  while  in  the  other 
three-fourths  of  her  diameter  was  north  of  the  limit;  that  is, 
one  fourth  of  the  moon's  disk  was  eclipsed.  It  was  thus  found 
that  the  moon's  apparent  diameter  was  Sl^',  and  the  appar- 
ent diameter  of  the  shadow  40f' .  The  former  number  was 
certainly  remarkably  near  the  truth.  From  tliis  it  was  con- 
cluded that  the  sun's  parallax  was  3'  11",  and  his  distance  1210 
radii  of  the  earth.  This  result  was  an  entire  mistake,  arising 
from  the  uncertainty  of  any  measure  of  so  small  an  angle. 
Really,  t  3  parallax  is  so  minute  as  to  elude  all  measurement 
with  any  instrument  in  which  the  vision  is  not  assisted  by  the 
use  of  a  telescope.  Yet  this  result  continued  to  figure  in  as- 
tronomy through  the  fourteen  centuries  during  which  the"yl/- 
magest''-  of  Ptolemy  was  tl»e  supreme  authority,  without,  appar- 


172  PRACTICAL  ASTRONOMY. 

ently,  any  astronomer  being  bold  enough  to  seriously  under- 
take its  revision. 

Kepler  and  his  contemporaries  saw  clearly  tliat  this  distance 
must  be  far  too  small ;  but  all  their  estimates  fell  short  of  the 
truth.  Wendell  came  nearest  tlie  truth,  as  he  claimed  that 
the  parallax  could  not  exceed  15".  But  the  best  estinuite  of 
the  seventeenth  century  was  made  by  lluyghens,*  the  reason 
why  it  was  the  best  being  that  it  v;as  not  founded  on  any 
attempt  to  measure  the  paralhix  itself,  which  was  then  real- 
ly incapable  of  measurement,  but  on  the  probable  magnitude 
of  the  earth  as  a  planet.  The  parallax  of  the  sun  is,  as  al- 
ready explained,  the  apparent  semidiameter  of  the  earth  as 
seen  from  the  sun.  If,  then,  we  can  find  what  size  the  earth 
would  aj)pear  if  seen  fi'om  the  sun,  the  problem  would  at  once 
be  solved.  The  apparent  magnitudes  of  the  planets,  as  seen 
from  the  earth,  are  found  by  direct  measurement  with  the 
telescope.  The  proportions  of  the  solar  system  being  known, 
as  already  explained,  it  is  very  easy  to  determine  the  magni- 
tudes of  all  the  ])lanets  as  seen  from  the  sun,  the  earth  alone 
excepted.  The  idea  of  lluyghens  was  that  the  earth,  being  a 
planet,  its  magnitude  would  probably  be  somewhere  near  that 
of  the  average  of  the  two  ])lanets  on  each  side  of  it,  namely, 
Venus  and  Mars.  So,  taking  the  mean  of  the  diameters  of 
Venus  and  Mars,  and  supposing  this  to  represent  the  diameter 
of  the  earth,  he  found  the  angle  which  the  semidiameter  of 
the  sup])()sed  earth  would  snbtend  from  the  sun,  which  would 
be  the  solar  parallax. 

Although  this  method  may  look  like  a  happy  mode  of 
guessing,  it  was  much  more  reliable  than  any  which  had  be- 
fore been  applied,  for  the  reason  that,  in  supposing  the  mag- 
nitude of  the  earth  to  be  between  those  of  Venus  and  Mars, 
he  was  likely  to  be  nearer  the  truth  than  any  measure  of  an 
angle  entirely  invisible  to  the  naked  eye  would  be.  And,  by 
a  lucky  accident,  Iluyghens's  estimate  was  nearer  the  truth 
than  any  determinations  made  previous  to  the  transit  of  Ve- 


*  At  the  close  of  his  "  Sptcma  Satiunium." 


MKASUKES  OF  THE  DISTANCE   OF  TUE  SIW.  173 

nus  ill  17G9,  his  result  for  the  distance  of  the  sun  being  25,08G 
semidiaineters  of  the  earth,  or  09  millions  of  miles.  If  he 
had  used  the  correct  diameters  of  Venus  and  Mars,  he  would 
have  been  farther  from  the  truth,  because  the  earth  is  consid- 
erably larger  than  the  mean  of  Venus  and  Mars — in  fact,  rath- 
er larger  than  Venus  herself.  But  the  imperfect  telescopes 
used  by  Iluyghens  showed  the  planets  larger  than  they  really 
were,  so  that  when  he  took  the  mean  diameter  of  these  planets 
as  they  appeared  in  his  telescopes,  he  just  hit  the  diameter  of 
the  earth,  and  reached  the  true  solution  of  the  problem. 

We  now  come  to  the  modern  methods  of  measuring  the 
parallax  of  the  sun.  These  consist,  not  in  measuring  this  par- 
allax directly,  because  this  cannot  even  now  be  done  with  any 
accuracy,  but  in  measuring  the  parallax  of  one  of  the  planets 
Venus  and  Mars  when  nearest  the  earth.  These  planets  pass- 
ing from  time  to  time  much  nearer  to  us  than  the  sun  does, 
have  then  a  much  larger  parallax,  and  one  which  can  easily 
be  measured.  Having  the  parallax  of  the  planet,  that  of  the 
sun  is  determined  from  the  known  proportion  between  their 
respective  distanoes. 

The  first  application  of  this  method  was  made  by  the  French 
astronomers  to  the  planet  Mars.  In  1G71  they  sent  an  ex- 
pedition to  the  colony  of  Cayenne,  in  South  America,  wdiicli 
made  observations  of  the  position  of  Mars  during  the  opposi- 
tion of  1672,  w^hile  corresponding  observations  were  made  at 
the  Paris  Observatory.  The  difference  of  the  two  ajiparent 
positions,  reduced  to  the  same  moment,  gave  the  parallax  of 
Mars.  From  a  discussion  of  these  observations,  Cassini  con- 
cluded the  parallax  of  the  sun  to  be  9".5,  corresponding  to  a 
distance  of  the  sun  equal  to  21, GOO  semidiameters  of  the  earth. 
This  distance  was  as  much  too  email  as  Iluyghens's  was  too 
great,  so  that,  as  we  now  know,  no  real  improvement  was 
made.  Still,  the  data  were  much  more  certain  than  those  on 
which  the  estimate  of  Iluyghens  was  made,  and  for  a  hundred 
years  it  was  generally  considered  that  the  sun's  parallax  was 
about  10",  and  his  distance  between  80  and  90  millions  of  miles. 

The  method  by  observations  of  Mars  is  still,  in  some  of  its 


174  PRACTICAL  ASTRONOMY. 

forms,  among  the  most  valuable  which  have  been  applied  to 
the  determination  of  the  sohir  parallax.  About  once  in  six- 
teen years  Mars  approaches  almost  as  near  the  i  cirth  as  Venus 
does  at  the  times  of  her  transits,  the  favorable  times  being 
those  when  Mai's  at  opposition  is  near  his  perihelion.  Ilis 
distance  outside  the  earth's  orbit  is  then  only  0.373  of  the  as- 
tronomical unit,  or  34^  millions  of  miles,  while  at  his  aphe- 
lion the  distance  is  nearly  twice  as  great.  At  the  nearest  op- 
positions, his  parallax  is  over  23",  an  angle  which  can  be  meas- 
ured with  some  accuracy.  The  plan  of  observation  has  gen- 
erally been  to  send  an  observing  party  to  the  southern  hemi- 
s})hero  in  advance,  for  the  purpose  of  making  observations  of 
the  position  of  Mars  on  the  celestial  sphere,  or  of  its  distance 
from  certain  selected  stars,  from  night  to  night,  while  corre- 
sponding observations  are  made  at  the  fixed  observatories  of 
the  northern  hemisphere.  The  displacement  of  the  planet 
due  to  parallax  is  then  found  by  comparing  the  results  of 
these  observations. 

The  last  expedition  of  this  sort  was  that  of  Captain  James 
M.  Gilliss,  late  of  the  United  States  Navy,  who  went  out  to 
Chili  under  the  auspices  of  the  American  Government  in 
1849,  and  remained  till  1852,  for  the  purpose  of  observing 
both  Venus  and  Mars  during  the  periods  vvhen  the  parallax 
was  greatest.  Several  circumstances  conspired  to  prevent  this 
enterprise  from  producing  results  corresponding  to  its  merits. 
The  opposition  of  Mars  was  a  very  unfavorable  one ;  observa- 
tions of  Venus  could  not  be  made  with  the  necessary  accu- 
racy, and  there  M^as  a  lack  of  sufficient  coopenition  on  the 
part  of  northern  observers.  The  astronomical  results  of  the 
expedition  were,  nevertheless,  important.  Captain  Gilliss  hav- 
ing prepared  an  immense  catalogue  of  the  stars  of  tlie  south- 
ern hemisphere,  while  his  instruments  became  the  property  of 
the  Government  of  Chili,  which  employed  them  in  fitting  up 
a  national  observacory.  Several  observatories  have  since  been 
founded  in  the  southern  hemisphere,  so  that  there  is  no  longer 
any  need  of  sending  out  expeditions  to  observe  the  planet 
Mars  for  the  purpose  in  question. 


SOLAli  PARALLAX  FliUM   TJUJS'SITS  OF   VENUS. 


175 


§  3.  Sohtr  Parallax  from  Transits  of  Venus. 

The  most  ccleb'-atod  iiietliod  of  determining  the  solar  paral- 
lax has  been  by  transits  of  Venus  ovjr  the  face  of  the  sun,  by 
which  the  difference  between  the  parallax  of  the  planet  and 
that  of  the  ='m  can  be  found,  as  explained  in  §  1.  We  know 
from  our  uotronomical  tables  that  this  phenomenon  has  recur- 
red in  a  certain  regular  cycle  four  times  every  243  years  for 
many  centuries  past.  This  cycle  is  made  up  of  four  intervals, 
the  lengths  of  which  are,  in  regular  order,  105^  years,  8  years, 
121^  years,  8  years,  after  which  the  intervals  repeat  tliem- 
selves.     The  dates  of  occurrence  for  eight  centuries  are  as 


follows : 

1518 June  2(1. 

lil'Jd June  1st. 

1 03 1 December  7tli. 

1G;51) December  'ith. 

17G! June  otb. 

17G9 JuneJkl. 

1874 December  9th. 


1882 December  Cth. 

2004 June8fl]. 

2012 JuneCth. 

2117 December  Uth. 

212.-, December  8th. 

2247 June  11th. 

2255 June'Jth. 


It  has  been  only  in  comparatively  recent  times  that  this  phe- 
nomenon could  be  predicted  anc^  observed.  In  the  years  1518 
and  152G  the  idea  of  looking  for  such  a  thing  does  not  seem 
to  have  occurred  to  any  one.  The  following  century  gave 
birth  to  Kepler,  who  so  far  improved  the  planetary  tables 
as  to  predict  tliat  a  transit  would  occur  on  December  Gth, 
1631.  But  it  did  not  co'^imence  until  after  sunset  in  Eu- 
rope, and  was  over  before  sunrise  next  morning,  so  that  it 
passed  entirely  unobserved.  Unfortunntely,  the  tables  were 
so  far  from  accurate  that  they  failed  to  indicate  the  transit 
which  occurred  eight  years  later,  and  led  Kepler  to  announce 
that  the  phenomenon  would  not  recur  till  1V61.  The  transit 
of  1G39  would,  therefore,  like  all  former  ones,  have  passed 
entirely  unobserved,  had  it  not  been  for  the  talent  and  enthu- 
siasm of  a  young  Englishman.  Jeremiah  Ilorrox  was  then  a 
young  curate  of  eight3en,  residing  in  the  North  of  England, 
who,  even  at  that  early  age,  was  a  master  of  the  astronomy  of 


176  PliACnCAL  ASTRONOMY. 

his,  times.  Comparing  different  tables  witli  his  own  observa- 
tions of  Venus,  lie  found  that  a  transit  might  be  expected  to 
occur  on  December  4th,  and  prepared  to  observe  it,  after  the 
fashion  then  in  vogue,  by  letting  the  image  of  the  sun  passing 
through  liis  telescope  fall  on  a  screen  behind  it.  Unfortu- 
nately, the  day  was  Sunday,  and  his  clerical  duties  prevented 
his  seeing  the  ingress  of  the  planet  upon  the  solar  disk — a  cir- 
cumstance which  science  has  mourned  for  a  century  past,  and 
will  have  reason  to  mourn  for  a  century  to  come.  AVhen  he 
returned  from  church,  he  was  overjoyed  to  see  the  planet  upon 
the  face  of  the  sun,  but,  after  following  it  half  an  hour,  the  ap- 
proach of  sunset  compelled  him  to  suspend  his  observations. 

During  the  interval  between  this  and  the  next  tran.  "t,  which 
occurred  in  17G1,  exact  astronomy  made  verj"^  rapid  p  ogress, 
through  the  discovery  of  the  law  of  gravitation  and  che  ap- 
plication of  the  telcsco})e  to  celestial  measurements.  A  great 
additional  interest  was  lent  to  the  phenomenon  by  Ilalley's 
discovery  that  observations  of  it  made  from  distant  points  of 
the  earth  could  be  used  to  determine  the  distance  of  the  sun. 

The  principles  by  which  the  parallaxes,  and  therefore  the 
distances,  of  Venus  and  the  sun  are  determined  by  Ilalley's 
method  are  quite  simple.  In  consequence  of  the  parallax  of 
Venus,  two  observers  at  distant  points  of  the  earth's  surface, 

watching    her    course    over   the 

O  solar  disk,  will  see  her  describe 
slightly  different  paths,  as  shown 
in  Fig.  50.  It  is  by  the  distance 
between  these  paths  that  the  par- 
allax has  hitherto  been  deter- 
mined. 
The  essential  princij)le  of  Ilal- 
ley's method  consists  in  the  mode 
Fig. (....-Apimrentpauis  of  Venus  ncro.8  of  determining  the  distaucc  be- 

the  sun,  as  seen  from  different  stntions  tWCCU  tllCSC  ai>parcnt  l)aths.       An 

duriufr  the  transit  of  1874.    Tlie  upper  .  .  p\       n  mi     i 

path  i.s  that  seen  from  a  southern  sta-  mspCCtlOn  ot  tllO  tlguro  Will  sllOW 

tion  :    the  h.wer  is  that  s-eeu  froni  a  ^^^^^   ^^^^      j^^j^   farthest   frOm    the 
northern  station,  out  the  distance  be-  '• 

tweeu  the  paths  is  exaggerated.  Sim's   CCUtrC   is    sllOrtcr   than   the 


SOLAR   PARALLAX  FROM  TRANSITS   OF  VENUS         177 

other,  so  that  Venus  will  pass  over  the  sun  more  quickly  when 
watched  from  a  southern  station  than  when  watched  from  a 
northern  one.  Ilalley  therefore  proposed  tha^  the  different  ob- 
servers should,  w^ith  a  telescope  and  a  chronometer,  note  the 
time  it  took  Venus  to  pass  over  the  disk,  and  the  diiference  be- 
tween these  times,  as  seen  from  different  stations,  would  give 
the  means  of  determining  the  difference  between  the  parallaxes 
of  Venus  and  the  sun.  The  ratio  between  the  distances  of 
the  planet  and  the  sun  is  known  with  great  exactness  by  Kep- 
ler's third  law,  from  which,  knowing  the  differences  of  paral- 
laxes, the  distance  of  eacli  body  can  be  determined. 

By  this  plan  of  Ilalley  the  observer  must  note  with  great 
exactness  the  times  both  of  beginning  and  end  of  the  transit. 
There  are  two  phases  which  may  be  observed  at  the  beginning 
and  two  at  tlie  end,  making  four  in  all. 

The  first  is  that  when  the  planet  iirst  touches  the  edge  of 
the  solar  disk,  and  begins  to  make  a  notch  in  it,  as  at  a,  Fig.  50. 
This  is  called  ^^5^  external  contact. 

The  second  is  that  when  the  planet  has  just  entered  entirely 
upon  the  sun,  as  at  b.     This  is  called  yi?".s/  internal  contact. 

The  third  contact  is  that  in  which  the  planet,  after  crossing 
the  sun,  lirst  reaches  the  edge  of  the  disk,  and  begins  to  go 
off,  as  at  c.     This  is  called  second  internal  contact. 

The  fourth  contact  is  that  in  which  the  planet  finally  disap- 
pears from  the  face  of  the  sun,  as  at  d.  This  is  called  second 
external  contact. 

Now,  it  M*as  the  opinion  of  Ilalley,  and  a  very  plausible  one, 
too,  that  the  internal  contacts  could  be  observed  with  far  great- 
er accuracy  than  the  external  ones.  He  founded  this  opinion 
on  his  own  experience  in  observing  a  transit  of  the  planet  Mer- 
cury at  St.  Helena  in  1677.  It  will  be  seen  by  inspecting  Fig. 
51,  which  represents  the  position  of  the  planet  just  before  first 
internal  contact,  that  as  the  planet  moves  forward  on  the  solar 
disk  the  sharp  horns  of  light  on  each  side  of  it  approach  each 
other,  and  that  the  moment  of  internal  contact  is  marked  by 
these  horns  meeting  each  other,  and  formiui;  a  thread  of  light 
all  the  way  across  the  dark  spac^e,  as  in  Fig.  52.     This  thread 

13 


178 


PRACTICAL  ASTRONOMY. 


of  light  is  indeed  simply  the  extreme  edge  of  the  snn's  disk 
coming  into  view  beliind  the  planet.  In  observing  the  tran- 
sit of  Mercury,  Ilalloy  felt 
sure  that  he  could  fix  the 
moment  at  which  the  horns 
met,  and  the  edge  of  the 
sun's  disk  appeared  un- 
broken, within  a  single  sec- 
ond ;  and  he  hence  con- 
cluded that  observers  of 
the  transit  of  Venus  could 
obser\e  the  time  required 

Fkj.  M.— Venus  appronchin;;  iuteniiil  contact  on  lOr  V  CUUS  tO  paSS  acrOSS 
the  face  of  th-  -.-.:-.  The  plauet  is  supposed  |.]jg  g^^^  ^yjthin  one  Or  tWO 
to  be  moving  upward. 

Seconds.  These  times  would 
differ  in  different  parts  of  the  earth  Dy  fifteen  or  twenty  min- 
utes, in  consequence  of  parallax.  Hence  it  followed,  that  if 
Ilalley's  estimate  of  the  de- 
gree of  accuracy  attainable 
were  correct,  the  parallax  of 
Venus  and  the  sun  would  be 
determined  by  the  proposed 
system  of  observations  within 
the  six  hundredth  of  its  whole 
amount. 

When  the  long-expected  5th 
of  June,  1701,  at  length  ap- 
proached, which  was  a  genei'- 
ation  after  ITallev's  death,  ex- 
peditioMs  were  sent  to  distant 
j>arts  of  the  worki  by  the  principal  European  nations  to  make 
the  required  observations.  The  French  sent  out  ti-om  among 
their  asti'onomei's,  Le  Gentil  to  Pondicherry;  Pingi'c  to  Rod- 
riguez Island,  in  the  neighboi-hood  of  the  Main'itius;  and  the 
Abbe  (^haj)pc  to  Tobolsk,  in  Siberia.  The  war  with  England, 
nnfoi'tunately,  prevented  the  first  two  from  reaching  their  sta- 
tions in  time,  but  Ciiappe  was  successful.     From  England,  Ma- 


Fiu.  62. — Internal  contact  of  the  limb  of  Ve- 
nus with  that  of  the  suu. 


SOLAK  PARALLAX  FROM   TRANSITS   OF  VENUS. 


179 


son — he  of  the  celebrated  Mason  and  Dixon's  Line — was  sent 
to  Sumatra;  but  he,  too,  was  stopped  by  the  war:  Maskelyne, 
the  Astronomer  Royal,  was  sent  to  St.  Helena.  Denmark, 
Sweden,  and  liussia  also  sent  out  expeditions  to  various  points 
in  Europe  and  Asia. 

With  those  observers  who  were  favored  by  fine  weather,  the 
entry  of  the  dark  body  of  Yen  us  upon  the  limb  of  the  sun 
was  seen  very  well  until  the  critical  moment  of  internftl  con- 
tact approached.  Then  they  were  perplexed  to  find  that  the 
planet,  instead  of  preserving  its  circular  form,  appeared  to 
assume  the  shape  of  a  pear  ;)r  a  balloon,  the  elongated  portion 
being  connected  with  the  limb  of  the  sun.  We  give  two  fig- 
ures, 52  and  53,  the  first  showing  how  the  planet  ought  to  have 
looked,  the  last  how  it  really  did  look.  Now,  we  can  readily 
see  that  the  observer,  looking 
at  sucii  an  appearance  as  in 
Fig.  53,  would  be  unable  to 
say  whether  internal  contact 
had  or  had  not  taken  place. 
The  round  part  of  the  planet 
is  entirely  within  the  sun,  so 
that  if  he  judged  from  this 
alone,  he  would  say  that  in- 
ternal contact  is  passed.  But 
the  horns  are  still  separated 
by  this  dark  elongation,  or 
"  black  drop,"  as  it  is  general- 
ly called,  so  that,  judging  from  this,  internal  contact  has  not 
taken  place.  The  result  was  an  uncertainty  sometimes  amount- 
ing to  nearly  a  minute  in  observations  which  were  expected  to 
be  correct  within  a  single  second. 

When  the  parties  returned  home,  and  their  observations 
were  computed  by  various  astronomers,  the  resulting  values 
of  the  solar  parallax  were  found  to  range  from  8".5,  found  by 
Short  of  England,  to  10".5,  found  by  Pingru,  of  Fraricc,  so 
that  there  was  nearly  as  nnich  uncertainty  as  ever  in  the  value 
of  the  element  sought.     Nothing  daunted,  however,  prepara- 


Fia.  Ba.— The  black  drop,  or  llgumeut. 


180  PRACTICAL  ASTRONOMY. 

tions  vet  moio  extensive  were  made  to  observe  tlie  transit  of 
1769.  Among  the  observers  was  one  whose  patience  and 
wliose  fortune  must  excite  our  warmest  sympatliies.  We  have 
said  that  Le  Gentil,  sent  out  by  the  French  Academy  to  ob- 
serve the  transit  of  1761  in  the  East  Indies,  was  i)revented 
from  reaching  his  station  by  the  war  with  England.  Finding 
the  first  port  he  attempted  to  reach  in  the  possession  of  the 
English,  his  commander  attempted  to  make  another,  and, 
meeting  with  unfavoralV  winds,  was  still  at  sea  on  the  day  of 
the  transit.  lie  thereupon  formed  the  resolution  of  remain- 
ing, with  his  instruments,  to  observe  the  transit  of  1760.  lie 
was  enabled  to  support  himself  by  some  successful  mercantile 
adventures,  and  he  also  industriously  devoted  himself  to  scien- 
tific observations  and  inquiries.  The  long-looked-for  morning 
of  June  4th,  1769,  found  him  thoroughly  prepared  to  make 
the  observations  for  which  he  had  waited  eight  long  years. 
The  sun  shone  out  in  a  cloudless  sky,  as  it  had  shone  for  a 
number  of  days  previously.  But  just  as  it  was  time  for  the 
transit  to  begin,  a  sudden  storm  arose,  and  the  sky  became 
covered  with  clouds.  When  they  cleared  away  the  transit 
was  over.  It  was  two  weeks  before  the  ill-fated  astronomer 
could  hold  tlie  pen  which  was  to  tell  his  friends  in  Paris  the 
story  of  his  disappointment. 

In  this  transit  the  ingress  of  Venus  on  the  limb  of  the  sun 
occurred  just  before  the  sun  was  setting  in  Western  Europe, 
which  allowed  numbers  of  observations  of  the  first  two  phases 
to  be  made  in  England  and  France.  The  commencement  was 
also  visible  in  this  country — which  was  then  these  colonies — 
under  very  favorable  circumstances,  and  it  was  well  observed 
by  the  few  astronomers  we  then  had.  The  leader  among 
these  was  the  talented  and  enthusiastic  Itittenhouse,  who  was 
already  well  known  for  his  industry  as  an  observer.  The  ob- 
servations were  organized  under  the  auspices  of  the  American 
Philosophical  Society,  then  in  the  vigor  of  its  youth,  and  par- 
ties of  observers  were  stationed  at  Norristown,  Philadeii)hia, 
and  Cape  Ilenlopen.  These  observations  have  every  appear- 
ance of  being  among  the  most  accurate  made  on  the  transit; 


SOLAR  PAR  Air,  AX  FROM   TRANSITS  OF  VENUS.         181 

but  they  have  not  received  the  consideration  to  which  they  are 
entitled,  partly,  we  suppose,  berause  the  altitude  of  the  sun 
was  too  great  to  admit  of  their  being  of  much  value  for  tlie 
determination  of  parallax,  and  partly  because  they  were  not 
very  accordant  with  the  European  observations. 

The  phenomena  of  the  distortion  of  the  planet  and  the 
"black  drop,"  already  described,  were  noticed  in  this,  as  in 
the  precading  transit.  It  is  strongly  indl..'\"e  of  the  ill 
preparation  of  the  observers  that  it  seems  to  have  taken  them 
all  by  surprise,  except  the  few  who  had  observed  the  preced- 
ing transit.  The  cause  of  the  appearance  was  first  pointed 
out  by  Lalande,  and  is  briefly  this :  when  we  look  at  i  bright 
object  on  a  dark  ground,  it  looks  a  little  larger  than  it  real- 
ly is,  owing  to  the  encroachment  of  tlie  light  upon  the  dark 
border.  This  encroachment,  or  irradiation,  may  arise  from  a 
number  of  causes — imperfections  of  the  eye,  imperfections  of 
the  lenses  of  the  telescope  when  an  instrument  is  used,  and 
the  softening  effect  of  the  atmosphere  when  we  look  at  a  ce- 
lestial object  near  the  horizon.  To  understand  its  effect,  we 
have  only  to  imagine  a  false  edge  painted  in  white  around  the 
borders  of  the  bright  object,  the  edge  becoming  narrower  and 
darker  where  the  bright  object  is  reduced  to  a  very  narrow 
line.  Thus,  by  painting  around  the  borders  of  the  light  por- 
tions of  Fig.  51,  we  have  formed  Fig.  53,  and  produced  an  ap- 
pearance cpiito  similar  to  that  described  by  the  observers  of 
the  transit.  The  bettor  the  telescope  and  the  steadier  the  at- 
mosphere, the  narrower  this  border  will  be,  and  the  more  the 
planet  will  seem  to  preserve  its  true  form,  as  in  Fig.  52.  In 
the  observations  of  the  recent  transit  of  Venus  with  the  im- 
proved instruments  of  the  present  time,  very  few  of  the  more 
experienced  observers  noticed  any  distortion  at  all. 

The  results  of  the  observations  of  1769  were  much  more 
accordant  than  those  of  1761,  and  seemed  to  indicate  a  paral- 
lax of  about  8".5.  Curious  as  it  may  seem,  more  than  half  a 
century  elapsed  after  the  transit  before  its  results  were  com- 
pletely worked  up  from  all  the  observations  in  an  entirely 
satisfactory  manner.     This  was  at  length  done  by  Encke,  in 


182  PBACTICAL  ASTRONOMY. 

1824,  for  both  transits,  the  result  giving  8",5776  for  the  solar 
parallax.  Some  suspicion,  however,  attached  to  some  of  the 
observations,  which  he  was  not  at  that  time  able  to  remove. 
In  1835,  having  examined  the  original  records  of  the  observa- 
tions in  question,  he  corrocted  his  work,  and  found  the  follow- 
ing separate  results  from  the  two  transits : 

raiallax  from  the  observations  of  ITfil 8".53 

Parallax  from  the  observations  of  17GI) S"J>'J 

Most  probable  result  from  both  transits 8". 571 

The  probable  error  of  the  result  was  estimated  at  0".037, 
which,  though  larger  than  was  expected,  was  much  less  than 
the  actual  error  has  since  proved  to  be.  The  corresponding 
distance  of  the  sun  is  95,370,000  miles,  a  classic  number 
adopted  by  astronomers  everywhere,  and  familiar  to  every 
one  who  has  read  any  work  on  astronomy. 

This  result  of  Encke  was  received  without  question  for 
more  than  thirty  years.  But  in  1854  the  celebrated  Hansen, 
completing  his  investigations  of  the  motions  of  the  moon, 
found  that  her  observed  positions  near  her  first  and  last  quar- 
ters could  not  be  accounted  for  except  by  supposing  the  par- 
allax of  the  sun  increased,  and  therefore  his  distance  dimin- 
ished, by  abont  a  thirtieth  of  its  entire  amount.  The  exist- 
ence of  this  error  has  since  been  amply  confirmed  in  several 
M'ays.  The  fact  is,  that  although  a  century  ago  a  transit  of 
Venus  afiorded  the  most  accurate  w^ay  of  obtaining  the  dis- 
tance of  the  sun,  yet  the  great  advances  made  during  the 
present  generation  in  the  art  of  observing,  and  the  applica- 
tion of  scientific  methods,  have  led  to  other  means  of  greater 
accuracy  than  these  old  observations.  It  is  remarkable  that 
while  nearly  every  class  of  observations  is  now  made  with 
a  precision  which  the  astronomers  of  a  century  ago  never 
thought  possible,  yet  this  particular  observation  of  the  interior 
contact  of  a  planet  with  the  limb  of  the  sun  has  never  been 
made  with  u-.y  thing  like  the  accuracy  which  ITalley  himself 
thou<2;ht  he  attained  in  his  observation  of  the  transit  of  Mer- 
curv  two  centuries  ago. 


SOLAR   PARALLAX  FROM  TRANSITS   OF  VENUS.         183 

The  knowledge  of  tiiis  error  in  the  fundamental  astronom- 
ical unit  gave  increased  interest  to  the  transit  of  Venus  which 
was  to  occur  on  December  8th,  1874,  The  rarity  of  the  phe- 
nomenon was  an  advantage,  in  that  it  led  to  an  amount  of 
public  interest  being  taken  in  it  which  could  not  have  been 
excited  by  any  otlier  astronomical  event,  and  thus  secured 
from  various  governments  the  grants  necessary  to  fit  out  the 
necessary  parties  of  observation.  Plans  of  observation  began 
to  be  worked  out  very  far  in  advance.  In  1857,  Professor 
Airy  sketched  a  general  plan  of  operations  for  the  observation 
of  the  transits,  and  indicated  the  regions  of  the  globe  in  whicli 
he  considered  the  observations  should  be  made.  In  1870,  be- 
fore any  steps  whatever  were  taken  in  this  country,  he  had  ad- 
vanced so  far  in  his  preparations  as  to  have  his  observing  huts 
all  ready,  and  his  instruments  in  process  of  construction.  In 
1869,  the  Prussian  Government  appointed  a  commission,  con- 
sisting of  six  or  eight  of  its  most  eminent  astronomers,  to  de- 
vise a  plan  of  operations,  and  report  it  to  the  Government 
with  an  estimate  of  the  expenses.  About  the  same  time  the 
Russian  Government  began  making  extensive  preparations 
for  observing  the  transit  from  a  great  number  of  stations  in 
Siberia. 

Active  pi-eparations  for  the  observations  in  question  were 
commenced  by  the  United  States  Govermnent  in  1871.  An 
account  of  the  niethod  of  observation  adopted  by  the  Com- 
mission to  whom  the  matter  was  intrusted  may  not  be  devoid 
of  interest.  The  observations  of  the  older  transits  havini!: 
failed  in  giving  results  of  the  accuracy  now  required,  it  be- 
came necessar^^  to  improve  upon  the  system  then  adopted. 
In  this  system,  the  parallax  depended  entirely  on  observations 
of  contacts,  the  uncertainty  of  which  we  have  already  shown. 
Besides  this  uncertainty,  Ilalley's  method  was  open  to  the  ob- 
jection that,  unless  both  contacts  were  observed  at  each  sta- 
tion, the  path  of  Venus  could  not  be  determined,  and  no  re- 
sult could  be  deduced.  It  was  therefore  proposed  by  De 
I'Isle  early  in  the  last  century,  that  the  observers  should  de- 
termine the  longitudes   of  their  stations,  in   order  that,  by 


184  PRACTICAL  ASTRONOMY. 

means  of  it.  they  could  find  the  actual  intervals  between  the 
moments  aL  wliich  any  given  contact  was  seen  at  the  different 
stations.  This  metliod  was  an  improvement  on  Ilalley's,  in 
that  it  diminished  the  chances  of  total  failure.  Still,  it  de- 
pended entirely  upon  making  an  accurate  observation  of  the 
moment  of  contact,  an  was  liable  to  fail  from  any  accident 
which  might  interfere  with  such  an  observation  —  a  passing 
cloud,  or  a  disarrangement  of  some  of  the  instruments  of  ob- 
servation. Besides,  it  was  not  yet  certain  whether  the  obser- 
vations could  be  made  with  the  necessary  accuracy.  It  was, 
therefore,  desirable  that,  instead  of  depending  on  contacts 
alone,  some  method  should  be  adopted  of  finding  the  position 
of  Venus  on  the  face  of  the  sun  as  often  as  possible  during 
the  four  hours  which  she  should  occupy  in  passing.  The 
easiest  and  most  effective  way  of  doing  this  seemed  to  be  to 
take  j)hotographs  of  the  sun  with  Venus  on  his  disk,  which 
photographs  could  be  \rought  home,  compared,  and  measured 
at  leisure. 

This  mode  of  astronomical  measurement  has  been  brought 
to  great  perfection  in  this  country  by  Mr.  L.  M.  Rutherfurd 
and  others,  and  has  been  found  to  give  results  exceeding  in 
accuracy  any  yet  attained  by  ordinary  eye  observations.  The 
advantages  of  the  photograj)hic  metliod  are  so  obvious  that 
there  could  be  no  hesitation  about  employing  it,  and,  so  far 
as  is  known,  it  was  applied  by  every  European  nation  which 
sent  out  parties  of  observation.  But  there  is  a  great  and 
essential  difference  between  the  methods  of  photographing 
adopted  by  the  Americans  and  by  most  of  the  Europeans. 
The  latter  seem  to  have  devoted  all  their  attention  to  the 
problem  of  securing  a  good  sharp  photograph,  taking  it  for 
granted  that  when  this  photograph  was  measured  there  would 
be  no  further  difficulty.  But  the  measurement  at  home  is 
necessarily  made  in  inches  and  fractions,  while  the  distance 
we  must  know  is  to  be  found  in  minutes  and  seconds  of  an- 
gular measure.  If  we  have  a  map  by  measurements  on  which 
we  desire  to  know  the  exact  distance  of  two  places,  we  must 
first  know  the  exact  scale  on  which  the  map  is  laid  down, 


SOLAR  PARALLAX  FROM  TRANSITS  OF  VENUS.         185 

with  a  degree  of  accuracy  corresponding  to  that  of  onr  meas- 
ures. Just  so  with  our  photographs  taken  at  various  parts  of 
tlie  globe.  We  must  know  the  scale  on  which  the  inuiges  are 
photographed  before  we  can  derive  any  conclusions  from  our 
measures.  While  the  determination  of  this  scale  with  suffi- 
cient precision  for  ordinary  purposes  is  quite  easy,  this  is  by 
no  means  the  case  with  a  problem  where  so  much  accuracy 
was  required,  so  that  hero  lay  the  greatest  difficulty  which  the 
photographic  method  offered. 

In  the  mode  of  photographing  adopted  by  the  Americans 
this  difficulty  was  met  by  using  a  telescope  of  great  length 
— nearly  forty  feet.  So  long  a  telescope  would  be  too  un- 
wieldy to  point  at  the  sun ;  it  was  therefore  fixed  in  a  hor- 
izontal position,  the  rays  of  the  sun  being  thrown  into  it  by  a 
mirror.  The  scale  of  the  picture  was  determined  by  actually 
measuring  the  distance  between  the  object-glass  and  the  pho- 
tograph-plate. Each  station  was  supplied  with  special  appa- 
ratus by  which  this  measurement  could  be  made  within  the 
hundredth  of  an  inch.  Then,  knowing  the  position  of  the  op- 
tical centre  of  the  glass,  it  is  easy  to  calculate  exactly  how 
many  inches  any  given  angle  will  srbtend  on  the  photograph- 
plate.  The  following  brief  description  of  the  apparatus  will 
be  readily  understood  by  reference  to  the  figures: 

The  object-glass  and  the  support  for  the  mirror  are  mount- 
ed on  an  iron  pier  extending  four  feet  into  ^he  ground,  and 
firmly  embedded  in  concrete.  The  mirror  is  in  a  frame  at 
the  end  of  an  inclined  cast-iron  axis,  which  is  turned  with  a 
very  slow  motion  by  a  simple  and  ingenious  piece  of  clock- 
work. The  inclination  of  the  axis  and  the  rate  of  motion  are 
so  adjusted  that,  notwithstanding  the  diurnal  motion  of  the 
sun  —  or,  to  speak  more  accurately,  of  the  earth  —  the  sun's 
rays  will  always  be  reflected  in  the  same  direction.  This  re- 
sult is  not  attained  with  entire  exactness,  but  it  is  so  near  that 
it  will  only  be  necessary  for  an  assistant  to  touch  the  screws 
of  the  mirror  at  intervals  of  fifteen  or  twenty  minutes  during 
the  critical  hours  of  the  transit.  The  reflector  is  simply  a 
piece  of  finely  polished  glass,  without  any  silvering  whatever. 


180 


PRACTICAL  ASTRONOMY. 


It  only  reflects  about  a  twentieth  of  the  sun's  light;  but  so  in- 
tense are  his  rays  that  a  j)liotograph  can  be  taken  in  less  than 
the  tenth  of  a  second.  The  polishing  of  this  mirror  was  the 
most  delicate  and  difticult  operation  in  the  construction  of 
the  apparatus,  as  the  slightest  deviation  from  perfect  flatness 
would  be  fatal.  For  instance,  if  a  straight  edge  laid  upon  the 
glass  should  touch  at  the  edges,  but  be  the  hundred- thou- 
sandth of  an  inch  above  it  at  the  centre,  tho  reflector  would 
be  useless.  It  might  have  seemed  hopeless  to  seek  for  such  a 
degree  of  accuracy,  had  it  not  been  for  the  confidence  of  the 
Commission  in  the  mechanical  genius  of  Alvan  Clark  &  Sons, 
to  whom  the  manufacture  of  the  apparatus  was  intrusted. 
The  mirrors  were  tested  by  observing  objects  through  a  tele- 
scope, first  directly,  and  then  by  reflection  from  the  mirror. 
If  they  were  seen  with  equally  good  definition  in  the  two 
cases,  it  would  show  that  there  were  no  irregularities  in  the 
surface  of  the  mirror;  while  if  it  were  either  concave  or  con- 
vex, the  focus  of  the  telescope  would  seem  shortened  or 
lengthened.     The  first  test  was  sustained  perfectly,  while  the 


I 


OCTAMCe 
>»■•'  AKO  A  FUCTION. 


•<-^ 


'r-'\,. 


Fui.  54 — Method  of  photogrnpliing  the  transit  of  Venns  uHed  by  the  French  and  Ameri- 
can observers,  and  by  Lord  Lindsay. 


SOLAIi  PARALLAX  FROM  TRANSITS  OF  VENUS.         187 

circles  of  convexity  or  concavity  indicated  by  the  changes  of 
focus  of  the  photographic  telescope  were  many  miles  iu  di- 
ameter. 

Immediately  in  front  of  the  mirror  is  the  object-glass.  The 
curves  of  the  lenses  of  which  it  is  formed  are  so  arranged  that 
it  is  not  perfectly  achromatic  for  the  visual  rays,  but  gives  the 
best  photographic  image.  Thirty -eight  feet  and  a  fraction 
from  the  glass  is  the  focus,  where  an  image  of  the  sun  about 
four  and  a  quarter  inches  in  diameter  is  formed.  Here  an- 
other iron  pier  is  firmly  embedded  in  the  ground  for  the  sup- 
port of  the  photographic  plate -holder.  This  consists  of  a 
brass  frame  seven  inches  square  on  the  inside,  revolving  on  a 
vertical  rod,  which  passes  through  the  iron  plate  on  top  of  the 
pier.  Into  this  frame  is  cemented  a  square  of  plate-glass,  just 
as  a  pane  of  glass  is  puttied  in  a  window.  The  glass  is  divided 
into  small  squares  by  very  line  lines  about  one-five-hundredth 
of  an  inch  thick,  which  were  etched  by  a  process  invented  and 
perfected  by  Mr.  W.  A.  Rogers,  of  the  Cambridge  Observatory. 
The  sensitive  plate  goes  into  the  other  side  of  the  frame,  and 
when  in  position  for  taking  the  photograph,  there  is  a  space 
of  about  one-eicrhth  of  an  inch  between  the  ruled  linos  and 
the  plate.  The  former  are,  therefore,  photographed  on  every 
picture  of  the  sun  wliich  is  taken,  and  serve  to  detect  any 
contraction  of  the  collodion  film  on  the  glass  plate. 

The  rod  on  which  the  plate-holder  turns,  and  the  frame  it- 
self, are  perforated  from  top  to  bottom  by  a  vertical  opening 
one-sixth  of  an  inch  in  diameter.  Tiirouijh  the  centre  of  this 
opening,  passing  between  the  ruled  plate  and  the  photograph 
plate,  hangs  a  plumb-line  of  very  fine  silver  wire.  In  every 
picture  of  the  sun  this  plumb-line  is  also  photographed,  and 
this  marks  a  truly  vertical  line  on  the  plate  very  near  the  mid- 
dle vertical  etched  line.  A  spirit-level  is  fixed  to  the  top  of 
tlie  frame,  and  serves  to  detect  any  changes  in  the  inclination 
of  the  ruled  lines  to  the  horizon. 

One  of  the  most  essential  features  of  the  arrano:ement  is 
that  the  photographic  object-glass  and  plate-holder  are  on  the 
same  level,  and  in  the  meridian  of  the  transit  instrument  with 


188  PRACTICAL  ASTRONOMY. 

which  the  time  is  detenniiied.  Tiie  central  ruled  line  on  the 
])late-huldcr  is  tliiis  used  as  a  meridian  mark  for  tlie  transit. 
Tile  ^reat  advautaii^e  of  this  arrangement  is,  that  it  permits 
the  angle  whicli  tlie  line  joining  the  centres  of  the  sun  and 
Venus  makes  with  the  meridian  to  be  determined  wu.i  the 
greatest  precision  by  means  of  the  image  of  the  plumb-line 
which  is  [>liotographed  across  the  picture  of  the  sun.* 

Although  the  contact  observations  were  not  wholly  relied 
on,  they  were  by  no  means  neglected.  On  the  contrary,  the 
greatest  pains  were  taken  to  avoid  the  sources  of  error  which 
caused  so  much  trouble  in  1709.  To  learn  wliat  these  errors 
probably  were,  and  to  practise  the  observers  in  making  their 
observations  so  as  to  avoid  them,  an  artificial  planet  was  con- 
structed to  move  over  an  artificial  representation  of  a  portion 
of  the  solar  disk  by  clock-wovk.  The  apparatus  was  mounted 
on  the  top  of  a  building  about  3300  feet  distant,  in  order  to 
give  the  effect  of  atmospheric  undulations  and  softening  of 
the  edges  of  the  planet.  The  planet  was  represented  by  a 
black  disk  one  foot  in  diameter,  which  made  its  apparent  mag- 
nitude the  same  as  that 
of  Venus  in  transit.  The 
Sim  was  rejiresented  by 
a  white  screen  behind 
the  artificial  Venus,  the 
portions  of  the  edge  of 

F.o.55.-Artiflcial  transit  of  Venns.  ^hc    disk    wlicrc    VcnUS 

entered  and  left  being  formed  by  the  sloping  edges  of  a  black 
triangle,  as  showm  in  the  figure.  There  was  no  need  of  a  rep- 
resentation of  the  entire  sun.  The  motion  was  so  regulated 
that  the  time  occupied  by  the  disk  in  passing  from  external  to 


*  The  method  of  photographing  the  sun  by  a  fixed  horizontal  telescope  witii  a 
reflector  in  front  of  it  is  believed  to  have  been  first  proposed  in  France  by  Captain 
Laiissedat.  It  was  independently  invented  by  the  late  J'rofessor  Wiiilock,  who 
put  it  into  actual  operation  at  tiie  Harvard  College  Observatory  in  18G9,  and,  so 
far  as  tlie  author  is  aware,  was  the  first  one  to  do  so.  It  was  employed  not  only 
by  the  American  observers,  but  by  the  French,  and  by  Lord  Lindsay,  M.P.,  of 
Scotland.  The  latter  gentleman  fitted  out  a  finely  equipped  expedition  at  his  own 
expense  to  observe  the  transit  of  Venus  at  the  Mauritius. 


SOLAB  PARALLAX  FROM  TRANSITS  Oi'  VENUS.         189 

'nternal  contact,  and  the  angle  its  motion  made  with  tlio  edges 
of  the  triangle,  were  the  same  as  they  would  be  in  the  actual 
transit  as  viewed  from  some  point  where  it  occurred  near  the 
zenith.  The  disk  was  put  at  such  a  height  that  it  was  only 
about  tlirco  minutes  from  internal  contact  at  ingress  to  inter- 
nal contact  at  egress,  instead  of  four  honrri. 

Tile  observations  of  this  instrument  have  thrown  mncli  light 
on  the  question  of  the  black  drop,  and  the  distortion  of  the 
planet  seen  in  former  transits  of  Venus,  which  have  been  al- 
ready described.  What  is  perhaps  yet  better,  it  has  enabled 
us  to  account  for  a  number  of  puzzling  and  discordant  a})pear- 
ances  described  by  the  observers.  Father  Hell's  black  drop, 
seen  before  the  limbs  were  in  contact;  the  formation  of  inter- 
nal contact  by  a  fine  line  of  light,  though  the  cusps  were  blunt, 
as  seen  at  Hudson  Bay  ;  Captain  Cook's  '^  atmosphere  "  around 
Venus,  and  his  curious  black  piece  cut  out  of  the  edge  of  the 
sun,  may  all  be  said  to  have  been  identified  nearly  enough  to 
judge  what  the  appearances  really  were  which  were  so  vari- 
ously described.  In  looking  at  the  artificial  planet  near  the 
moment  of  internal  contact,  when  the  air  is  not  still,  the  first 
thing  which  the  observer  sees  is  that  there  is  really  no  con- 
stant shape  to  those  parts  of  Venus  and  the  sun  which  are  ap- 
proaching each  other;  but  that,  owing  to  the  undulations  of 
the  air,  they  assume  all  sorts  of  shapes  in  rapid  succession,  so 
that  different  observers  may  give  diffei'ent  descriptions  of  the 
appearances  presented,  though  looking  at  the  very  same  ob- 
ject. In  the  varied  forms  which  may  be  seen,  we  recognize 
all  tlie  peculiar  appearances  described  by  the  observers  of  the 
transit  of  1769. 

At  each  American  station  the  scientific  corps  consisted  of 
a  chief  of  party,  an  assistant  astronomer,  and  three  photog- 
raphers. The  instruments  at  all  the  stations  were  precisely 
similar,  and  the  operations  and  observations  the  same  at  all. 
This  system  was  adopted  to  secure  two  great  advantages:  first, 
to  run  the  least  risk  of  entire  failure  from  bad  weather;  and, 
second,  to  have  all  the  observations  strictly  comparable.  Much 
pains  and  trouble  were  devoted  to  these  objects.     To  appreci- 


190  PRACTICAL  ASTRONOMY. 

ate  their  importance,  we  must  remember  that,  in  order  to  de- 
duce the  parallax  from  the  observations  at  any  two  stations, 
it  is  essential  that  the  difference  between  observations  should 
be  due  only  to  parallax,  and  that  in  every  other  respect  they 
should  be  exactly  the  same;  because,  if  there  are  other  dif- 
ferences which  we  cannot  certainly  allow  for,  our  calculation 
of  the  parallax  will  be  wrong.  It  is  also  necessary  that  we 
compare  the  same  kind  of  observations  in  order  to  get  the 
parallax.  To  show  how  the  chances  of  failure  are  lessened, 
suppose  we  have  two  stations  in  each  hemisphere,  in  one  of 
which  eye  observations  are  made,  while  in  the  other  photo- 
graphs are  taken.  Then,  if  the  photographs  in  one  hemi- 
sphere and  the  eye  observations  in  the  other  are  lost  by  clouds, 
or  any  other  cause,  everything  will  be  lost,  although  one  sta- 
tion in  each  hemisphere  is  successful,  because  the  eye  obser- 
vations in  the  one  hemisphere  cannot  be  compared  with  the 
photographs  in  the  other.  It  being  decided,  for  these  reasons, 
to  have  the  same  system  of  observations  at  all  the  stations,  it 
became  ne-^'essary  to  confine  the  choice  of  stations  to  points 
where  the  entire  transit  would  be  visible. 

One  of  the  mo^t  important  features  of  the  preparations, 
which  distinguishes  them  from  the  preparations  t:>  observe 
the  former  transits,  was  the  previous  training  of  the  observers. 
All  the  members  of  the  observing  parties  assembled  at  Wash- 
ington to  practise  together  before  leaving  to  make  the  obser- 
vations. Tliey  took  all  their  multitudinous  instruments  and 
apparatus  out  of  their  boxes,  mounted  them,  and  proceeded  to 
practise  with  them  in  the  same  way  they  were  to  be  used  at 
the  stations.  Photographs  of  the  sun  were  taken  from  day  to 
day  in  the  same  way  as  on  the  8th  of  December,  and  each 
chief  of  party  was  instructed  in  all  the  delicate  operations 
necessary  to  secure  the  entire  success  of  his  operations. 

To  know  where  a  party  could  be  sent,  it  had  first  to  be 
known  when  and  where  the  transit  would  be  visible.  We 
give  a  small  map  of  the  world  showing  this  at  a  glance. 
Could  we  have  seen  the  planet  Venus  from  tlie  Eastern  States 
on  the  afternoon  of  December  8th,  1874,  we  should  have  seen 


SOLAS  PARALLAX  FROM  TRANSITS  OF  VENUS.         191 


Pia.  66 Map  of  the  earth,  showing  the  areas  of  visibility  of  the  transit  of  1874. 

her  approaching  iioarer  and  nearer  the  sun  as  the  latter  ap- 
proached the  horizon.  In  San  Francisco,  where  sunset  is  three 
hours  later  than  here,  she  would  have  been  so  near  the  sun  as 
almost  to  seem  to  touch  it.  About  an  hour  later  she  actual- 
ly reached  the  solar  disk.  The  sun  was  then  shining  on  the 
whole  Pacific  Ocean,  except  that  portion  nearest  the  Ameri- 
can coast,  and  on  Eastern  Asia,  Australia,  and  the  Indian  and 
Antarctic  oceans  to  the  south  pole.  Venus  was  about  four 
and  a  half  hours  passing  over  the  face  of  the  sun,  and  during 
this  time  the  latter  had  set  across  the  entire  northern  portion 
of  the  Pacific  Ocean,  and  had  risen  as  far  west  as  Moscow 
and  Vienna,  from  which  cities  the  planet  might  have  been 
seen  to  leave  the  disk  just  as  the  sun  rose. 

In  the  northern  hemisphere  suitable  stations  were  easily 
found,  as  we  have  the  whole  of  China,  Japan,  and  Northern 
India.  But  in  the  southern  hemisphere  great  difhculties  were 
encountered,  owing  to  the  want  of  habitable  stations  in  the 
regions  which  were  astronomically  the  most  favorable.  Ob- 
servations cannot  be  made  from  the  deck  of  a  ship;  astrono- 
mers must  have  solid  ground  for  their  instruments.  The  south 
pole  would  have  been  the  best  station  of  all,  if  some  antarc- 
tic Kane  or  Hall  could  take  a  party  thither.  The  antarctic 
continent  and  the  neighboring  islands  were  not  to  be  thought 
ofj  because  a  party  could  neither  be  landed  nor  subsisted  there : 


192  PRACTICAL  ASTRONOMY. 

and  if  they  could,  the  weather  would  probably  have  prevented 
any  observations  from  being  taken.  The  chance  of  having  a 
clear  sky  on  the  eventful  8tli  of  December  was,  indeed,  one 
of  the  most  important  considerations  on  which  the  choice  of 
a  station  had  to  depend.  Information  from  every  available 
source,  official  and  private,  respecting  the  meteorology  of  the 
various  possible  stations,  was  therefore  sought.  Where  there 
was  any  American  consul  or  consular  agent,  he  was  applied 
to  through  the  State  Department  to  have  meteorological  ob- 
servations made  during  the  months  of  November  and  Decem- 
ber, 1872  and  1873.  A  sealing  ship  belonging  to  the  lirm  of 
Williams,  Haven,  &  Co.,  of  New  London,  made  observations 
at  Heard's  Island,  in  the  Southern  Indian  Ocean.  From  all 
these  reports,  as  well  as  f'om  the  printed  reports  issued  by 
various  authorities,  it  was  found  that  the  chances  of  good 
weather  were  much  better  in  the  northern  than  in  the  south- 
ern hemisphere.  In  consequence,  instead  of  sending  an  equal 
number  of  parties  north  and  soutli,  it  was  determined  to  send 
three  to  the  northern  and  five  to  tlie  southern  hemisphere. 

The  stations  wliich  the  American  parties  finally  occupied, 
with  the  names  of  the  chiefs  of  party,  are  as  follows : 

NOUTIIKUN  IIemisi'iiere. 

Wladiwostok,  Siberia Prof      )r  Asaph  Hall,  U.  S.  N. 

Pekin,  Ciiina Protesi-or  J.  C.  Watson. 

Nagasaki,  Japan Professor  George  Davidson,  U.  S.  Coast  Survey. 

Southern  Stations. 

Kergnelcn  Island Commander  G.  P.  Rvan,  U.  S.  N. 

IIoI)art-to\vn,  Tasmania Professor  W.  IIarkness,  U.  S.  N. 

Canipbelltown,  Tasmania* Captain  C.  W.  Kaymono,  Engineer  Corps,  U.  S.  A. 

Queenstowii,  New  Zealand.  ...Professor  C.  II.  F.  Peters. 

Chatham  Island Edwin  Smith,  Esq.,  U.  S.  Coast  Survey. 

The  southern  parties  were  all  carried  to  tlieir  respective  sta- 
tions by  the  U.  S.  steamer  Swatara,  Captain  Kalph  Chandler, 
U.  S.  N.,  commanding. 

*  Captain  Haymond's  party  was  designed  for  the  Crozet  Islands,  but  the  Swa- 
tara  failed  to  efl'ect  a  landing  there. 


SOLAR  PARALLAX  FROM  TRANSITS  OF  VENUS.         198 

Tlie  only  thing  wliieli  seriously  interfered  with  the  observa- 
tions was  the  weather.  Some  photographs  were  obtained  at 
every  station,  but  the  full  number  at  none.  Altogether,  there 
were  only  about  half  the  expected  riuniber  obtained.  No 
contacts  at  all  were  observed  at  llobart-towii  or  Chatham  Isl- 
and, but  one  or  more  were  observed  at  each  of  the  remaining 
six  stations.  Pekin  was,  however,  the  only  one  at  which  all 
four  were  observed.  Among  the  parties  sent  out  by  other 
nations,  the  most  fortunate,  as  regards  weather,  were  the  Ger- 
mans, who  were  successful  at  all  six  of  their  stations.  The 
English,  French,  and  Russians  were,  on  the  average,  about  as 
successful  as  the  Americans. 

If  the  observations  on  the  transit  of  1874  had  been  made 
in  the  same  way  as  those  of  the  transit  of  176D,  they  could  be 
very  speedily  worked  up,  and  we  shouM  soon  expect  to  sec 
the  solar  parallax  deduced  from  the  combination  of  them  all. 
But  the  investigation  and  measurement  of  the  photographs  is 
so  laborious  an  operation  that  the  American  results  can  hard- 
ly be  published  before  1878.  The  definitive  value  of  the 
parallax  must  then  be  deduced,  not  from  the  observations  of 
any  one  nation,  but  so  far  as  possible  from  the  combination 
of  those  of  all  nations.  We  must,  therefore,  wait  for  the  final 
publication  and  discussion  of  all  the  observations  before  the 
definitive  value  of  the  parallax  can  be  announced. 

Under  these  circumstances,  the  question  whether  it  is  worth 
while  to  send  out  parties  to  observe  the  transit  of  1882  will 
soon  be  a  subject  of  discussion  atnong  astronomers,  the  answer 
to  which  will  de}iend  very  largely  on  the  success  of  the  efforts 
made  in  1874.  On  this  success  we  cannot  pronounce  a  final 
judgment  until  all  the  observations  are  worked  up.  The  rea- 
son why  doubt  still  remains  on  this  point  is  that  the  sun  is  a 
very  difficult  object  either  to  observe  or  to  photograph  with 
accuracy,  owing  to  the  action  of  his  rays  on  the  atmosphere. 
The  air  near  the  ground  becomes  heated,  and  thus  causes  the 
limb  of  the  sun  to  undulate  to  a  degree  which  sometimes  ren- 
ders its  exact  definition  out  of  the  question,  while  the  outline 
of  Venus  undulates  in  the  same  way.     Another  difliculty  is, 

14 


194  PRACTICAL  ASTRONOMY. 

that  tlie  iri'egnlarity  in  the  transparency  of  the  atmosphere, 
ovvino^  to  clouds  and  vapors,  renders  the  pliotographic  repre- 
sentation of  the  limb  of  the  sun  quite  uncertain,  and  thus  re- 
quires all  measures  to  be  made  from  the  sun's  centre.  Now, 
we  cannot  sav  how  far  these  difficulties  have  been  surmount- 
ed  by  the  methods  of  observation  adopted  until  we  tinally 
compare  all  the  observations,  and  see  how  consistent  they  are 
with  each  other:  and  this  (;annot  be  done  for  several  vears. 

The  region  of  visibility  of  the  transit  of  1882  will  be  quite 
different  from  that  of  1874,  as  it  will  include  the  whole  Amer- 
ican continent,  except  some  portions  in  or  near  the  arctic  <;ir- 
cle.  The  beginning  will  be  visible  over  a  large  part  of  Afri- 
ca, and  the  end  over  most  of  the  Pacific  Ocean.  The  most 
favorable  northern  stations  for  its  observation  are  in  the  East- 
ern and  Middle  States. 

§  4.    OOier  Methods  of  determining  the  Suns  Distance^  and  their 

Results. 

The  methods  of  determininc;  the  astronomical  unit  wl  ich 
we  have  described  rest  entirely  upon  measures  of  parallax,  an 
angle  which  hardly  ever  exceeds  20",  and  which  it  is  there- 
fore exceedingly  difficult  to  measure  with  the  necessary  ac- 
curacy. If  there  were  no  other  way  than  this  of  determining 
the  sun's  distance,  we  might  despair  of  being  sure  of  it  with- 
in 200,000  miles.  But  the  refined  investigations  of  modern 
science  have  brou<rht  to  li<«;ht  other  methods,  bv  at  least  two 
of  which  we  may  hope,  ultimately,  to  attain  a  greater  degree 
of  accuracy  than  we  can  by  measuring  parallaxes.  Of  tliese 
two,  one  depends  on  the  gravitating  force  of  the  sun  upon  the 
moon,  and  the  other  upon  the  velocity  of  light. 

Parallactic  Equation  of  the  Moon. — The  motion  of  the  moon 
around  the  earth  is  largely  affected  by  the  gravitating  force 
of  the  sun,  or,  to  speak  more  exacth',  by  the  difference  of  the 
gravitating  force  of  the  sun  upon  the  moon  and  upon  the 
earth.  A  part  of  this  difference  depends  upon  the  proj)ortion 
between  the  respective  distances  of  the  moon  and  the  sun,  so 
that  when  this  force  is  known,  the  proportion  can  be  deter- 


METHODS  OF  DETERMINING   THE  SUN'S  DISTANCE.     197 

mined.  The  distance  of  the  moon  being  known  with  nil  nec- 
essary precision,  we  have  only  to  multiply  it  by  the  proportion 
thus  obtained  to  get  the  distance  of  the  sun.  The  force  in 
question  shows  itself  by  producing  a  certain  inequality  in  the 
moon's  motion,  by  which  she  falls  two  minutes  behind  her 
mean  place  near  the  lirst  quarter,  and  is  two  minutes  ahead 
near  her  last  quarter.  In  determining  this  inequality,  we  have 
to  measure  an  angle  about  six  times  as  great  as  the  average 
of  the  planetary  parallaxes  on  which  the  sun's  distance  de- 
pends ;  so  that,  if  we  could  measure  both  angles  with  the  same 
precision,  the  error,  by  using  the  moon,  would  be  only  one- 
sixth  as  great  as  in  direct  measures  of  parallax.  But  it  seems 
as  if  nature  had  determined  to  allow  mankind  no  royal  road 
to  a  knowledge  of  the  sun's  distance.  It  is  the  position  of 
the  moon's  centre  which  we  require  for  the  purpose  in  ques- 
tion, and  this  can  never  be  directly  fixed.  We  have  to  make 
our  observations  on  the  limb  or  edge  of  the  moon,  as  illu- 
minated by  the  sun,  and  must  reduce  our  observations  to  the 
moon's  centre,  before  we  can  use  them.  The  worst  of  the 
matter  is,  tnat  one  limb  is  observed  at  the  first  quarter,  and 
another  at  the  third  quarter,  so  that  we  cannot  tell  Avith  abso- 
lute certainty  how  much  of  tlie  observed  inequality  is  real, 
and  how  much  is  due  to  the  change  from  one  limb  to  the  other. 
So  great  is  the  uncertainty  here  that,  previous  to  1854,  it  was 
supposed  that  the  inequality  in  question  was  about  122", 
agreeing  with  the  theoretical  inequality  from  Encke's  erroH'- 
ous  value  of  the  solar  parallax.  Hansen  then  found  that  it 
was  really  about  4"  greater,  and  thus  was  led  to  the  conclusion 
that  the  parallax  of  the  sun  must  be  increased,  and  his  distance 
diminished,  by  one-thirtieth  of  the  whole  amount. 

It  is  quite  likely  that  by  adopting  improved  modes  of  ob- 
servation, it  will  be  found  that  the  sun's  distance  can  be  more 
accurately  measured  in  this  way  than  through  the  parallaxes 
of  the  planets.  Some  pains  have  already  been  taken  to  deter- 
mine the  exact  amount  of  the  inequality  from  observations, 
the  result  being  125".5.  Tlie  entire  seconds  may  here  be  re- 
lied on,  but  the  decimal  is  quite  uncertain.     We  can  only  say 


198  PRACTICAL  ASTRONOMY. 

that  we  are  pretty  surely  within  three  or  four  tentlis  of  a  sec- 
ond of  tlie  truth.  From  this  vahie  the  paralUix  of  the  sun  is 
found  to  be  8".83,  with  an  uncertainty  of  two  or  three  hun- 
dredths of  a  second. 

Sun's  Distance  from  the  Velocity  of  Ligld.  —  There  is  an  ex- 
traordinary beauty  in  this  method  of  measuring  the  sun's  dis- 
tance, arising  from  the  contrast  between  the  simplicity  of  the 
principle  and  the  profoundness  of  the  methods  by  which  alone 
the  principle  can  be  applied.  Suppose  we  had  a  messenger 
whom  we  could  send  to  and  fro  between  the  sun  p  1  the 
earth,  and  who  could  tell,  on  his  return,  exactly  how  long  it 
took  him  to  perform  his  journey;  suppose,  also,  we  knew  the 
exact  rate  of  speed  at  which  he  travelled.  Then,  if  we  mul- 
tiply his  speed  by  the  time  it  took  him  to  go  to  the  sun,  we 
shall  at  once  have  the  sun's  distance,  just  as  we  could  deter- 
mine the  distance  of  two  cities  when  we  knew  that  a  train 
running  thirty  miles  an  hour  required  seven  hours  to  pass  be- 
tween them.  Such  a  messenger  is  light.  It  has  been  found 
practicable  to  determine,  experimentally,  about  how  fast  light 
travels,  and  to  find  from  astronomical  phenomena  how  long 
it  takes  to  come  from  the  sun  to  the  earth.  How  these  de- 
terminations are  made  will  be  shown  in  the  next  chapter; 
here  we  shall  stop  only  to  givv^  results.  It  is  found  by  Fou- 
cault's  experiment  that  light  travels  about  185,200  miles  per 
second ;  and  it  is  known  from  a  study  of  several  astronomical 
phenomena  that  it  passes  from  the  sun  to  the  earth  in  498  sec- 
onds. The  product  of  these  numbers  gives  a  distance  of 
92,230,000  miles,  a  result,  however,  which  is  uncertain  by  -j-J^ 
of  its  entire  amount,  or  nearly  half  a  million  of  miles,  owing  to 
the  uncertainty  in  each  of  the  factors.  This  result  was  reached 
in  1862,  and  was  one  of  the  first  confirmations  of  the  increased 
value  of  the  solar  parallax  found  by  Hansen.  But  since  that 
time  a  redetermination  of  the  velocity  of  light  has  been  made 
by  Cornu,  of  Paris,  by  a  method  soon  to  be  described,  with  a 
different  result.  He  finds  a  velocity  of  300,400  kilometres  or 
186,670  miles  per  second,  making  the  distance  of  the  sun 
92,960,000  miles,  and  its  parallax  8".794.     This  discrepancy 


METHODS  OF  DETEUMINING   THE  SUN'S  DISTANCE.     199 

is  not  yet  explained,  and  the  truth  can  be  readied  only  by  a 
repetition  of  one  or  both  of  the  experiments. 

These  two  methods  of  determining  the  distance  of  the  sun 
may  fairly  be  regarded  as  equal  in  accuracy  to  that  by  tran- 
sits of  Venus  when  they  are  employed  in  the  best  manner. 
There  are  also  two  or  three  minor  methods  which,  though 
less  accurate,  are  worthy  of  mention.     One  of  the  most  in- 
genious of  these  was  first  applied  by  Leverrier.     It  is  known 
from  the  theory  of  gravitation  that  the  earth,  in  consequence 
of  the  attraction  of  the  moon,  describes  a  small  monthly  orbit 
around  the  common  centre  of  gravity  of  these  two  bodies,  cor- 
responding to  the  monthly  revolution  of  the  moon  around  the 
earth,  or,  to  speak  with  more  precision,  around  the  same  com- 
mon centre  of  gravity.     If  we  know  the  mass  (or  weight)  of 
the  moon  relatively  to  that  of  the  earth,  and  her  distance,  we 
can  thus  calculate  the  radius  of  the  little  orbit  referred  to. 
In  round  numbers,  it  is  3000  miles.     This  monthly  oscillation 
of  the  earth  will  cause  a  corresponding  oscillation  in  the  lon- 
gitude of  the  sun,  and  by  measuring  its  apparent  amount  we 
can  tell  how  far  the  sun  must  be  placed  to  make  this  amourt 
correspond  to,  say  3000  miles.     Leverrier  found  the  oscilla- 
tions in  arc  to  be  6^.50,     From  this  he  concluded  the  solar 
parallax  to  be  8''.95.     But  Mr.  Stone,*  of  Greenwich,  found 
two  errors  in  Leverrier's  computation, f  and,  when  these  are 
corrected,  the  result  is  reduced  to  8''.85, 

Another  recondite  method  has  been  employed  by  Leverrier. 
It  is  founded  on  the  principle  that  when  the  relative  masses 
of  the  sun  and  earth  are  known,  their  distance  can  be  found 
by  comparing  the  distance  which  a  heavy  body  will  fall  in 
one  second  at  the  surface  of  the  ■  arth  with  the  fall  of  the  lat- 
ter towards  the  sun  in  the  same  time.  The  mass  of  the  earth 
was  found  by  its  disturbing  action  on  the  planets  Venus  and 
Mars,  as  explained  in  the  chapter  on  Gravitation.     Leverrier 

*  Mr.  E.  J.  Stone  was  tlien  first  assistant  at  the  Royal  Observatory,  Green- 
wich, but  has  been  Astronomer  Royal  at  the  Cape  of  Good  Hope  since  1870. 

+  "Monthly  Notices  of  the  Royal  Astronomical  Society,"  vol.  xxvii.,  p.  241, 
and  vol.  xxviii.,  pp.  22,  23. 


200  rUAVTICAL   ASTRONOMY. 

concluded  that  tliis  method  gave  the  value  of  the  solar  paral- 
lax as  8". 80.  But  one  of  his  numbers  requires  a  small  correc- 
tion, which  reduces  it  to  8".83.  Another  determination  of  the 
mass  of  the  earth  relative  to  that  of  the  sun  has  recently  been 
made  by  Von  Asten,  of  Pulkowa,  fi-om  the  action  of  the  earth 
upon  Encke's  comet.  The  solar  parallax  thence  resulting  is 
9".009,  the  largest  recent  value ;  but  the  anomalies  in  the  ap- 
parent motions  of  this  comet  are  such  that  very  little  reliance 
can  be  placed  upon  this  result. 

Yet  another  method  of  determining  the  solar  parallax  has 
been  proposed  and  partially  carried  out  by  Dr.  Galle.*  It 
consists  in  measuring  the  pai'allax  of  some  of  the  small  plan- 
ets between  Mars  and  Jupiter  at  the  times  of  their  nearest 
approach  to  the  earth,  by  observations  in  the  northern  and 
southern  hemispheres.  The  least  distance  of  the  nearest  of 
these  bodies  from  us  is  little  less  than  that  of  the  sun,  so  that 
in  this  respect  they  are  far  less  favorable  than  Venus  and 
Mars.  But  they  have  the  great  advantage  of  being  seen  in 
the  telescope  only  as  points  of  light,  like  stars,  and,  in  conse- 
quence, of  having  their  position  relative  to  the  surrounding 
stars  determined  with  greater  precision  than  can  be  obtained 
in  the  case  of  disks  like  those  of  Venus  and  Mars.  Observa- 
tions of  Flora  were  made  in  this  way  at  a  number  of  observa- 
tories in  both  hemispheres  during  the  opposition  of  1874,  from 
which  Dr.  Galle  has  deduced  8".875  as  the  value  of  the  solar 
parallax. 

Most  Probable  Value  of  the  Sun's  Parallax. — From  the  gen- 
eral accordance  of  the  various  methods  we  have  described,  it 
would  appear  that  the  solar  parallax  must  lie  between  pretty 
narrow  limits,  probably  between  8".82  and  8".86,  and  that 
the  distance  of  the  sun  in  miles  probably  lies  between  the 
limits  92,200,000  and  92,700,000.  Of  the  distance  of  tlie 
sun,  we  may  say  with  a  reasonable  approach  to  certainty  that 
it  is  92,000,000  and  some  fraction  of  another  million ;  and 

*  Dr.  J.  G.  Galle,  row  director  of  the  observatory  at  Breslaii,  Eastern  Prussia. 
He  was  formerly  assistant  at  tiie  Observatory  of  Berlin,  where  be  became  cele- 
brated as  tlie  optical  discoverer  of  the  planet  Neptune. 


STELLAR    PARALLAX.  201 

if  we  sliould  guess  that  fraction  to  be  400,000,  we  should 
probably  be  within  200,000  miles  of  the  truth.  This  is  all 
we  can  say  of  the  sun's  distance  until  the  results  of  the  tran- 
sits of  Venus  are  obtained,  when  we  may  hope  .u  lind  the 
uncertainty  brought  between  yet  narrower  limits. 

In  many  recent  works  the  distance  in  cpiestion  will  be  found 
stated  at  91,000,000  and  some  fraction.  This  arises  from  the 
circumstance  that  into  several  of  the  first  determinations  by 
the  new  methods  small  errors  and  imperfections  crept,  which, 
by  a  singular  coincidence,  all  tended  to  make  the  parallax  too 
great,  and  therefore  the  distance  too  small.  For  instance, 
Hansen's  original  conipntations  from  the  motion  of  the  moon 
led  him  to  a  parallax  of  8".96.  Revising  his  calculations,  he 
j-educed  it  to  8".917.  When  his  lunar  tables,  published  in 
1857,  came  to  be  compared  with  observations,  it  was  found 
that  his  parallactic  inequality  was  undoubtedly  too  great  by 
one  second  or  more.  When  this  is  corrected,  the  parallax  is 
reduced  about  a  tenth  of  a  second  more. 

The  observations  of  Mars,  in  1862,  as  reduced  by  Winnecke 
and  Stone,  first  led  to  a  parallax  of  8".92  to  8".94.  But  in 
these  investigations  only  a  small  portion  of  the  observations 
was  used.  When  the  great  mass  remaining  was  joined  with 
them,  the  result  was  8". 85. 

The  early  determinations  of  the  time  required  for  light  to 
come  from  the  sun  were  founded  on  the  extremely  uncertain 
observations  of  eclipses  of  Jupiter's  satellites,  and  were  five  to 
six  seconds  too  small.  The  time,  493  seconds,  being  used  in 
some  computations  instead  of  498  seconds,  the  distance  of  the 
sun  from  the  velocity  of  light  was  made  too  small. 

In  both  of  Leverrier's  methods  some  small  errors  of  computa- 
tion have  been  found,  the  effect  of  all  of  which  is  to  make  his 
parallax  too  great.  Correcting  these,  and  making  no  change  in 
any  of  his  data,  the  results  are  respectively  8".85  and  8".83. 

§  5.  Stellar  Parallax. 

It  is  probable  that  no  one  thing  tended  more  strongly  to 
impress  the  minds  of  thoughtful  men  in  former  times  with 


202  PRACTICAL  ASTRONOMY. 

the  belief  tliat  flic  earth  was  immovablr  than  did  the  absence 
of  stellar  parallax.  We  may  call  to  mind  that  the  annual  })ar- 
allax  of  the  fixed  stars  arises  from  the  chance  in  their  direc- 
tion  produced  by  the  motion  of  the  earth  from  one  side  of 
its  orbit  to  the  other.  One  of  the  earliest  forms  in  which  we 
may  suppose  this  parallax  to  have  •  ^en  looked  for  is  sliown 
in  Fig.  58.     Snp})ose  AB  to  be  the  earth's  orbit  with  the  sun, 

li    A  T 


Fio.  Cs.— Effect  of  stellar  parallax. 

A',  near  its  centre,  and  RI'  two  stars  so  situated  as  to  be  dii-ect- 
ly  opposite  eacli  other  wben  the  earth  is  at  A;  that  is,  when 
the  direction  of  each  star  is  90°  distant  from  that  of  the  sun. 
Then  it  is  clear  that,  after  six  months,  when  the  earth  is  at  B^ 
the  stars  will  no  longer  be  opposite  each  other,  the  point  ?7, 
which  is  opposite  R,  making  the  angle  TBU,  with  the  direc- 
tion of  T.  The  stars  will  all  be  dis[)laced  in  the  same  direc- 
tion that  the  sun  is  in  from  the  earth.  When  it  was  found 
that  the  most  careful  observations  showed  no  such  displace- 
ment, the  conclusion  that  the  earth  did  not  move  seemed  in- 
evitable. We  have  seen  how  Tycho  Nvas  led  in  this  way  to 
reject  the  doctrine  of  the  earth's  motion,  and  favor  a  system 
in  which  the  sun  moved  around  it.  In  this  Tycho  was  fol- 
lowed by  the  ecclesiastical  astronomers  who  lived  during  the 
seventeenth  century,  and  who,  finding  no  parallax  whatever  to 
any  of  the  stai's,  were  led  to  reject  the  Copernican  system. 

The  telescope  furnishing  so  powerful  an  auxiliary  in  meas- 
uring small  angles,  it  was  natui'al  that  the  defenders  of  the 
Copernican  system  should  be  anxious  to  employ  it  in  detect- 
ing the  annual  parallax  of  the  stars.  But  the  earlier  observ- 
ers had  very  imperfect  notions  of  the  mechanical  appliances 
necessar}'  to  do  this  with  success,  and,  in  consequence,  the  in- 
vention of  the  telescope  did  not  result  in  any  immediate  im- 


STELLAR   PARALLAX.  203 

provement  in  the  inctliods  of  celestial  ineasurcmciit.  A  t^tep 
was  taken  in  1GG9  by  Ilooku,  of  England,  who  was  amon<^  the 
first  to  see  how  the  teleseo[)e  wac  to  he  applied  i)i  the  nieas- 
urement  of  the  ai)parent  distances  of  the  stars  from  the  ze- 
nith, lie  fixed  a  telescope  thirty-six  feet  long  in  his  house,  in 
a  vertical  position,  the  object-glass  being  in  an  opening  in  the 
roof,  while  the  eye-piece  was  in  one  of  the  lower  rooms.  A 
line  i)lumb-line  luuig  down  from  the  object-glass  to  a  point 
below  the  eye  piece,  which  gave  a  truly  vertical  line  from 
which  to  measure.  The  star  selected  for  observation  was  y 
Draconis,  because  it  was  comparatively  bright,  and  passed  ovei 
the  zenith  of  London.  His  mode  of  observation  was  to  meas- 
ure the  distance  of  the  image  of  the  star  from  the  plumb-line 
from  d£.;  to  day  at  the  moment  of  its  passing  the  meridian, 
lie  had  made  but  four  observations  when  his  object-glass  was 
accidentally  broken,  and  the  attempt  ended  without  leading 
to  any  result  whatever. 

Between  1701  and  1704:,  Roemer,  then  of  Copenhagen,  at- 
tempted to  determine  the  sum  of  the  double  parallaxes  of 
Sirius  and  o  Lyne  by  the  principle  shown  in  Fig.  58.  These 
stars  lie  somewhere  near  the  opposite  quarters  of  the  celestial 
sjjlicre,  and  the  angle  between  them  will  vary  from  spring  to 
autunni  by  nearly  double  the  sum  of  their  parallaxes.  The 
angle  was  measured  by  the  transit  instrument  and  the  astro- 
nomical clock,  by  noting  the  time  which  ela})sed  between  the 
transit  of  Sirius  over  the  meridian,  and  that  of  a  Lyrse.  This 
time  wr.s  found  to  be,  on  the  average, 

}lr8.     Min.       Sec. 

In  February,  Marcli,  .nnd  April 11     TA    .lO.  7 

In  September  anil  October 11     M     ">'). 4 

Difference 4.3 

Here  was  a  difference  of  four  seconds  of  time,  or  a  minute  of 
angle,  which  was  then  very  naturall}'  attributed  to  the  motion 
of  the  earth,  and  wdiich  was  afterwards  printed  in  a  disserta- 
tion entitled  "  Copernicus  Triumphans."  It  is  now  known  that 
there  is  no  such  parallax  as  this  to  either  of  these  stars,  and 


204  PRACTICAL  ASTRONOMY. 

Peters*  has  sliown  that  the  difference  which  was  attributed 
to  paralhix  by  the  enthusiastic  Danish  astronomers  really  arose, 
in  great  part,  from  the  diurnal  irregularity  in  the  rate  of  their 
clock,  caused  by  the  action  of  the  diurnal  change  of  tempera- 
ture upon  the  uncompensated  pendulums.  In  the  spring  the 
interval  of  time  measured  elapsed  during  the  night,  Sirius 
passing  the  meridian  in  the  evening,  and  a  Lyraj  in  the  morn- 
ing. The  cold  of  night  made  tlie  clocks  go  too  fast,  and  so 
the  measured  interval  came  out  too  great.  In  the  autumn 
Sirius  passed  in  the  morning,  and  a  Lyra3  in  the  evening ;  the 
clock  was  going  too  slow  on  account  of  the  heat  of  the  day, 
and  the  interval  came  out  too  small. 

Among  the  numerous  other  vain  efforts  made  by  the  astron- 
omers of  the  last  century  to  detect  the  stellar  parallax,  that  of 
Bradley  is  worthy  of  note,  owing  to  the  remarkable  discovery 
of  the  aberration  of  light  to  which  it  led.  The  principle  of 
his  instrument  was  th.e  same  as  that  of  Ilooke,  the  zenith  dis- 
tance of  the  star  j  Dracor^s  at  the  moment  of  its  passing  the 
meridian  being  determined  by  the  inclination  of  a  telescope  to 
a  fine  plumb-line.  The  instrument  thus  used,  which  has  be- 
come so  celebrated  in  the  history  of  astronomy,  has  since  been 
known  as  Bradley's  zenith  sector.  In  accuracy  it  was  a  long- 
step  in  advance  of  any  which  preceded  it,  so  that  by  its  means 
Bradley  was  able  to  announce  with  certainty  that  the  star  in 
question  had  no  parallax  approaching  a  single  second.  But 
he  found  another  annual  oscillation  of  a  very  remarkable 
character,  arising  from  the  progressive  motion  of  light,  which 
will  be  described  in  the  next  chapter.  It  lias  fi-equently  hap- 
pened in  the  history  of  science  that  an  investigation  of  some 
cause  has  led  to  discoveries  in  a  different  direction  of  an  en- 
tirely unexpected  character. 

It  would  be  tedious  to  describe  in  detail  all  the  efforts 
made  by  astronomers,  during  the  last  century  and  the  early 
part  of  the  present  one,  to  detect  the  stellar  parallax.     It  will 


*  C.  A.  F.  Peters,  tlieii  of  the  Pulkowa  Observnlory,  nnd  now  editor  of  tlie  As- 
trunomische  Nachrichten. 


STELLAR  PARALLAX.  205 

be  sufficient  to  say,  in  a  general  way,  that  they  depended  on 
absolute  measures;  that  is,  the  astronomer  endeavored,  gen- 
erally by  a  divided  circle,  to  determine  from  day  to  day  the 
zenith  distance  at  which  the  star  passed  tlie  meridian.  The 
position  of  the  zenith  was  determined  in  various  ways — some- 
times by  a  fine  plumb-line,  sometimes  by  tlie  level  of  quick- 
silver. What  is  required  is  the  angle  between  the  plumb-line 
and  the  line  of  sight  from  the  observer  to  the  star.  The  same 
result  can  be  obtained  by  observing  the  angle  between  a  ray 
coming  directly  from  a  star  and  the  ray  wliich,  coming  from 
the  star,  strikes  the  surface  of  a  basin  of  quicksilver,  and  is  re- 
flected upwards.  Whatever  method  is  used,  a  large  angle  has 
to  be  measured,  an  opei-ation  which  is  always  affected  by  un- 
certainty, owing  to  the  influences  of  varying  temperatures  and 
many  other  causes  upon  the  instrument.  The  general  result 
of  all  the  efforts  nuide  in  this  way  was  that  while  several  of 
the  brighter  stars  seemed  to  some  astronomers  to  have  paral- 
laxes, sometimes  amounting  to  two  or  three  seconds,  though 
generally  not  much  exceeding  a  second,  yet  there  was  no  such 
agreement  between  the  various  results  as  was  necessary  to  in- 
spire confidence.  As  a  matter  of  fact,  we  now  know  that 
these  results  were  entirely  illusory,  being  due,  not  to  parallax, 
but  to  the  unavoidable  errors  of  the  instruments  used. 

Struve  was  the  first  one  to  prove  conclusively  that  the  par- 
allaxes even  of  the  brighter  stars  were  so  small  as  to  abso- 
lutely elude  every  mode  of  measurement  before  adopted.  In 
principle  h;s  method  was  that  empl  d  by  Roemer,  the  sum 
of  the  parallaxes  of  stars  twelve  hours  distant  in  right  ascen- 
sion being  determined  bv  the  annual  change  in  the  intervals 
between  their  times  of  transit  over  the  meridian.  But  he 
made  the  great  inq^rovtment  of  selecting  stars  which  could 
be  observed  as  they  passed  the  meridian  below  the  pole,  as 
well  as  above  it,  so  that  a  short  time  before  or  after  observing 
the  transit  of  a  star  he  could  turn  his  transit  instrument  be- 
low the  pole,  and  observe  the  transit  of  the  opposite  star  from 
west  to  east.  Thus  lie  was  not  under  the  necessity  of  depend- 
ing on  the  rate  of  his  clock  for  more  than  an  hour  or  two. 


206  PRACTICAL   ASTRONOMY. 

while  Roemer  had  to  depend  on  it  for  twelve  hours.  The  re- 
sult of  Struvo  was  tliat  the  average  parallax  of  the  twenty- 
five  brightest  stars  within  45°  of  the  pole  could  not  much,  if 
at  all,  exceed  a  single  tenth  of  a  second. 

Such  was  the  general  state  of  things  up  to  the  year  1835. 
It  was  then  decided  by  Struve  and  Bessei,  in  lieu  of  attempt- 
ing to  determine  zenith  distances,  to  adopt  the  method  of 
relative  parallaxes.  The  idea  of  this  method  really  dates  al- 
most from  the  invention  of  the  telescope.  It  was  considered 
by  Galileo  and  Iluyghens  that  where  a  bright  and  a  faint 
star  were  seen  side  by  side  in  the  field  of  view  of  a  telescope, 
the  latter  was  probably  vastly  more  distant  than  the  former, 
nnd  that  consequently  they  would  change  their  relative  po- 
sition as  the  earth  moved  from  one  side  of  the  sun  to  the  oth- 
er. If,  for  instance,  one  star  was  three  times  the  distance  of 
the  other,  its  apparent  motion  produced  by  parallax  would  be 
only  a  third  that  of  the  other,  and  there  would  remain  a  rel- 
ative parallax  equal  to  two-thirds  that  of  the  brighter  star, 
which  could  be  detected  by  measuring  the  angular  distance 
of  the  two  stars  as  seen  in  the  telescope  from  day  to  day 
throughout  the  year.  The  drawback  to  which  this  method  is 
subject  is  the  impossibility  of  determining  how  many  times 
farther  the  one  star  is  than  the  other ;  in  fact,  it  may  be  that 
tlie  smaller  star  is  really  no  farther  than  the  large  one.  No 
doubt  it  was  this  consideration  which  deterred  tlie  astrono- 
mers of  the  last  century  from  trying  this  very  simple  method. 

The  astronomers  of  the  last  generation  found  cases  in 
which  there  could  be  little  doubt  that  a  star  was  much  near- 
er to  us  than  the  small  stars  which  surrounded  it  in  the  field 
of  the  i:".i  .scope.  For  instance,  the  star  Gl  Cygni,  or  rather 
the  pair  of  stars  thus  designated,  are  found  not  to  occupy  a 
fixed  position  in  the  celestial  sphere,  like  tlie  surrounding 
small  stars,  but  to  he  moving  forward  in  a  straight  line  at  the 
rate  of  six  seconds  per  year.  This  amount  of  proper  motion 
was  so  unusual  as  to  make  it  probable  that  the  star  must  be 
one  of  the  nearest  to  us,  although  it  was  only  of  the  sixth  nuig- 
nitude.     It  was  therefore  selected  by  iJessel  for  the  investi- 


STELLAR   PARALLAX.  207 

gation  of  its  paiallax  relative  to  two  other  stars  in  its  neigli- 
borliood.  The  iiistriiinent  used  was  the  helionieter,  an  in- 
strument whieli,  as  now  made,  admits  of  great  precision,  but 
which  was  then  liable  to  small  uncertainties  from  various 
causes.  Ilis  early  attempts  to  detect  a  parallax  failed  as 
completely  as  had  those  of  former  obsefvers.  He  recom- 
menced them  in  August,  1837,  his  first  series  of  measures  be- 
in"-  continued  until  October,  1838.  The  result  of  this  series 
was  the  detection  of  a  parallax  of  about  three-tenths  of  a  sec- 
ond (0''.3136).  He  then  took  down  his  instrument,  nnide  some 
improvements  in  it,  and  connnenced  a  second  series,  which  lie 
continued  until  July,  1839;  and  his  assistant,  Schluter,  until 
March,  1840.  The  final  value  of  the  parallax  deduced  by 
Bessel  from  all  these  observations  was  0".35.  The  reality  of 
this  paralUix  has  been  well  established  by  subsequent  investi- 
gators, only  it  has  been  found  to  be  a  little  larger.  From  a 
combination  of  all  the  results,  Auwers,  of  Berlin,  finds  tlie 
most  probable  parallax  to  bo  0".51. 

The  star  selected  by  Struve  for  the  measure  of  relative  par- 
allax was  the  bri<>'ht  one  a  Lvrae.  This  has  not  only  a  sensible 
projjcr  motion, but  is  of  the  first  magnitude;  so  that  there  is 
every  reason  to  believe  it  to  be  among  those  which  are  nearest 
to  us.  The  comparison  was  made  with  a  single  very  small 
star  in  the  neighborhood,  the  instrument  used  being  the  nine- 
inch  telescope  of  the  Dorpat  Observatory.  The  observations 
extended  from  November,  1835,  to  August,  1838.  The  result 
was  a  relative  parallax  of  a  quarter  of  a  second.  Subsequent 
investigations  liave  reduced  this  purallax  to  two-tenths  o.  a 
second,  so  that  although  a  Lyra3  is  nearly  a  hundred  times  as 
bright  as  either  of  the  pair  of  stars  CI  Cygni,^t  is  more  tlum 
twice  as  far  from  us. 

So  far  as  is  known,  and,  beyond  all  reasonable  doubt,  in  re- 
ality, the  nearest  fixed  star  is  a  Centauri,  in  the  southern  liem- 
isphere.  This  fact  was  discovei'cd  by  Henderson,  the  English 
Astronomer  Royal  at  the  Cape  of  Good  Hope,  about  tlie  same 
time  that  Struve  and  Bessel  were  making  their  first  measures 
of  paraUaxes.      The  observations  on   which  it  was  founded 


208  PRACTICAL  ASTEONOMY. 

were  made  with  the  mural  circle  of  tlie  Cape  Observatory, 
and  were  therefore  absolute  measures  of  zenith  distance,  in- 
stead of  comparisons  with  surrounding  stars,  like  the  measures 
of  Struve  and  Bessel.  From  a  discussion  of  his  own  observa- 
tions, and  a  very  careful  series  by  his  successor,  Hender- 
son found  tlie  parallax  of  the  pair  of  stars  which  compose 
a  Centauri  to  be  0".91.*  This  parallax  corresponds  to  the 
distance  of  226,000  astronomical  units,f  or  more  tlian  twenty 
millions  of  millions  of  miles.  Yet  it  is  not  only  the  nearest 
star,  but  so  far  the  nearest  that  no  other  is  known  to  be  with- 
in nearly  double  the  distance. 

Tlie  most  elaborate  measures  of  stellar  parallax  made  in 
recent  times  are  those  by  Dr.  Briiimow,  formerly  director  of 
the  observatory  at  Ann  Arbor,  Michigan.  On  his  appointment 
to  the  post  of  Astronomer  Royal  for  Ireland,  Dr.  Briinnow 
employed  the  equatorial  telescope  of  the  Dnnsink  Observa- 
tory in  such  determinations  with  great  success.  The  results 
of  his  measures,  with  those  of  other  astronomers,  are  given  in 
the  Appendix  to  the  present  work. 

The  recent  researches  of  various  observers  have  resulted  in 
showing  that  there  are  about  a  dozen  stars  visible  in  our  lati- 
tudes of  which  the  parallax  ranges  from  a  tenth  to  half  a  sec- 
ond. Part  of  these  are  small  stars,  supposed  to  be  near  us 
from  their  large  proper  motion,  while  others  are  stars  of  the 
far  brighter  classes.  It  is,  however,  remarkable  that  among  the 
thirteen  stars  of  the  first  magnitude  visible  in  our  latitudes, 
less  than  half  have  been  found  to  have  any  measurable  paral- 
lax, even  when  the  greatest  refinements  have  been  applied  in 
the  observations.  For  the  most  part,  the  stars  with  a  decided 
parallax  are  not  of  a  conspicuous  magnitude.  The  two  stars 
next  in  distance  to  a  Centauri  are  61  Cygni,  of  the  fifth  mag- 
nitude, and  one  in  Ursa  Major  without  a  name,  and  too  small 

*  The  mean  of  all  tlie  measures  of  the  parallax  of  this  jwir  of  stars  hitherto 
made,  gives  0".1)3  as  their  most  probable  parallax,  corresitonding  to  a  distance 
of  22I,0()0  astronomical  units. 

t  The  astronomical  unit  is  the  distance  of  the  earth  from  the  sun,  about  02j 
millions  of  miles. 


STELLAR  PARALLAX.  209 

to  be  seen  without  a  telescope.  Tlie  parallax  of  the  latter  has 
been  found  by  Professor  Winnecke*  to  be  0''.501,  which  is 
nearly  the  same  as  that  of  61  Cygni.  Tlie  question  of  the 
average  distance  of  the  stars  of  tlie  first  magnitude  must 
thei-efore  be  regarded  as  still  uusolved.  We  can  only  say 
that  the  parallax  of  at  least  half  of  them  is  probably  less  than 
the  tenth  of  a  second,  and,  therefore,  the  distance  greater  than 
two  million  radii  of  the  earth's  orbit.f 

In  these  measurements  of  the  annual  parallax  of  the  fixed 
stars,  it  sometimes  happens  that  the  astronomer  finds  his  ob- 
servations to  give  a  nerjative  parallax.  To  understand  what 
this  means,  we  remark  that  a  determination  of  the  distance  of 
a  star  is  made  by  determining  its  directions,  as  seen  from  op- 
posite points  of  the  earth's  orbit.  If  we  draw  a  line  from 
each  of  these  points,  in  the  observed  direction  of  the  star,  the 
point  in  which  the  lines  meet  marks  the  position  of  the  star. 
A  negative  parallax  shows  that  the  two  lines,  instead  of  con- 
verging to  a  point,  actually  diverge,  so  that  there  is  no  pos- 
sible position  of  the  star  to  correspond  to  the  observations. 
Such  a  paradoxical  result  can  arise  only  from  errors  of  obser- 
vation. 

*  Dr.  A.  Winnecke,  formerly  assistant  at  the  Pulkowa  Observatory,  and  now 
director  of  tlie  observatory  at  Strasbiirg. 

t  A  list  of  tlie  stars  of  wliicli  the  parallaxes  have  been  determined  will  be  found 
in  the  Appendix. 

15 


210  rUACTlCAL  ASIR02s'0MY. 


CHAPTEE  ly. 

THE   MOTION   OF    LIGHT. 

Intimately  connected  with  celestial  measurements  are  the 
curious  phenomena  growing  out  of  tlie  progressive  move- 
ment of  light.  It  is  now  known  that  when  we  look  at  a  star 
we  do  not  see  the  star  that  now  is,  but  the  star  that  was  sev- 
eral years  ago.  Though  the  star  sliould  suddenly  be  blotted 
out  of  existence,  we  should  still  see  it  shining  for  a  number 
of  years  before  it  would  vanish  from  our  sight.  We  should 
see  an  event  that  was  long  past,  perhaps  one  that  was  past 
before  we  were  born.  This  non-coincidence  of  the  time  of 
perception  with  that  of  occurrence  is  owing  to  the  fact  tliat 
light  requires  time  to  travel.  We  can  see  an  object  only  by 
light  which  emanates  from  it  and  reaches  our  eye,  and  tluis 
our  sight  is  behind  time  by  the  interval  required  for  the  light 
to  travel  over  the  space  which  separates  us  from  the  ol)ject. 

It  was  by  observations  of  the  satelh'tes  of  Jupiter  that  it 
was  first  found  that  celestial  phenomena  were  thus  seen  be- 
hind time.  These  bodies  revolve  round  Jupiter  much  more 
rapidly  than  our  moon  does  around  tlie  earth,  the  inner  satel- 
lite making  a  complete  revolution  in  eighteen  hours.  Owing 
to  the  great  magnitude  of  Jupiter  and  his  sliadow,  tliis  satel- 
lite, as  also  the  two  next  outside  of  it,  are  eclipsed  at  every  rev- 
olution. The  accuracy  with  which  the  times  of  disappearance 
in  the  shadow  could  be  observed,  and  the  consequent  value  of 
such  observations  for  the  determination  of  longitudes,  led  tlie 
astronomers  of  the  seventeenth  century  to  make  tables  of  the 
times  of  occurrence  of  these  eclipses.  In  attempting  to  im- 
prove the  tables  of  his  predecessors,  it  was  found  by  Roemer 
(then  of  Paris,  though  a  Dane  by  birth)  that  the  times  of  the 


THE  MOTION  OF  LIGHT.  211 

eclipses  could  not  be  represented  by  an  equable  motion  of 
the  satellites.  lie  could  easily  represent  the  times  of  the 
eclipses  when  Jupiter  was  in  opposition  to  the  sun,  and  there- 
fore tlie  earth  nearest  to  .lupiter.  But  then,  as  the  earth  re- 
ceded from  Jupiter  in  its  annual  course  round  the  sun,  the 
eclipses  were  constantly  seen  later,  until,  when  it  was  at  its 
greatest  distance  from  Jupiter,  the  times  appeared  to  be  22 
minutes  late.  Such  an  inetpuility,  Roemer  concluded,  could 
not  be  real ;  he  therefore  attributed  it  to  the  fact  that  it  must 
take  time  for  light  to  come  from  Jupiter  to  the  earth,  and 
that  this  time  is  greater  the  more  distant  the  earth  is  from 
the  planet.  lie  therefore  concluded  that  it  took  light  22 
minutes  to  cross  the  orbit  of  the  earth,  and,  consequently,  11 
minutes  to  come  from  the  sun  to  the  earth. 

The  next  great  step  in  the  theory  of  the  progressive  motion 
of  light  was  made  by  the  celebrated  Bradley,  afterwards  As- 
tronomer Royal  of  England,  to  whose  observations  at  Kew  on 
the  star  y  Draconis  with  his  zenith  sector,  in  order  to  deter- 
mine the  parallax  of  the  star,  allusion  has  already  been  made. 
The  effect  of  parallax  w^ould  have  been  to  make  the  declina- 
tion greatest  in  Ju!ie  and  least  in  December;  while  in  March 
and  Septeml)er  the  star  would  occupy  an  intermediate  or 
mean  position.  But  the  actual  result  of  the  measures  w^as 
entirely  diiferent,  and  exhibited  phenomena  which  Bradley 
could  not  at  first  account  for.  The  declinations  of  June  and 
December  were  the  same,  showing  no  effect  of  ])arallax.  But, 
instead  of  remaining  the  same  the  rest  of  the  year, the  decli- 
nation was  some  forty  seconds  greater  in  Se])tember  than  in 
March,  when  the  effect  of  parallax  should  be  the  same.  Tims, 
the  star  had  a  regular  annual  oscillation  ;  but  instead  of  its 
apparent  motion  in  this  little  orbit  being  opposite  to  that  of 
the  earth  in  its  annual  orbit,  as  required  by  the  laws  of  rela- 
tive motion,  it  was  constantl}'  at  right  angles  to  it. 

After  long  consideration,  Bradley  saw  the  cause  of  the 
phenomenon  in  the  progressive  motion  of  light  combined 
with  the  motion  of  the  earth  in  its  orbit.  In  Fig.  50  let  S 
be  a  star,  and   OT  a  telescope  pointed  at  it.     Then,  if  the 


212 


PRACTICAL  ASTRONOMY. 


s^  ^s' 


Q 


telescope  is  not  in  motion,  the  ray  SOT  emanating  from  the 
star,  and  entering  the  centre  of  the  object-glass, 
will  pass  down  near  the  rigl it-hand  edge  of  the  eye- 
piece, and  the  star  will  appear  in  the  right  of  the 
Held  of  view.  But,  instead  of  being  at  rest,  all  our 
telescopes  are  carried  along  with  the  earth  in  its 
orbit  round  the  sun  at  the  rate  of  nearly  nineteen 
miles  a  second.  Suppose  this  motion  to  be  in  the 
direction  of  the  arrow ;  then,  while  the  ray  is  pass- 
ing down  the  telescope,  the  latter  moves  a  short  dis- 
tance, so  that  the  ray  no  longer  strikes  the  right- 
hand  edge  of  the  eye-piece,  but  some  point  farther 
to  the  left,  as  if  the  star  were  in  the  direction  S\ 
and  the  ray  followed  the  course  of  the  dotted  line. 
In  order  to  see  the  star  centrally,  the  eye  end  of  the 
"Cr  telescope  must  be  dropped  a  little  behind,  so  that, 
Fio.  59.  —  instead  of  pointing  in  the  direction  *^S',  it  will  really 
Aberration  x^q  pointing  in  tlic  direction  ^iS",  shown  by  the  dotted 
ray.  This  will  then  represent  the  apparent  direc- 
tion of  the  star,  which  will  seem  displaced  in  the  direction  in 
whicli  the  earth  is  moving. 

The  phenomenon  is  quite  similar  to  that  presented  by  the 
apparent  direction  of  the  wind  on  board  a  steamship  in  mo- 
tion. If  the  wind  is  really  at  right  angles  to  the  course  of  the 
ship,  it  will  appear  more  nearly  ahead  to  those  on  board ;  and 
if  two  ships  are  passing  each  other,  they  will  appear  to  have 
the  wind  in  different  directions.  Indeed,  it  is  said  to  have 
been  through  noticing  this  very  result  of  motion  on  board  a 
boat  on  the  Thames,  that  the  cause  of  the  phenomenon  he 
had  observed  was  suggested  to  Bradley. 

The  displacement  of  the  stars  which  we  have  explained  is 
called  the  Aberration  of  Li fj] it.  Its  amount  depends  on  the  ra- 
tio of  the  velocity  of  the  earth  in  its  orbit  to  the  velocity  of 
light.  It  can  be  determined  by  observing  the  declination  of 
a  star  at  the  proper  seasons  during  a  number  of  years,  by 
which  the  annual  displacement  will  be  shown.  The  value 
now  most  generally  received  is  that  determined  by  IStruve  at 


THE  MOTION  OF  LIGHT.  213 

the  Pulkowa  Observatory,  and  is  20".445.  Thoni^h  this  is  the 
inost  rehable  vahie  yet  found,  tlie  two  last  figures  are  both 
uncertain.  We  can  say  little  more  than  that  the  constant 
probably  lies  between  20^.43  and  20".48,  and  that,  if  outside 
these  limits  at  all,  it  is  certainly  very  little  outside. 

This  amount  of  aberration  of  each  star  shows  that  light 
travels  10,089  times  as  fast  as  the  earth  in  its  orbit.  From 
this  we  can  determine  the  time  lio;ht  takes  to  travel  from  the 
sun  to  the  earth  entirely  independent  of  the  satellites  of  Ju- 
piter. The  earth  makes  the  circuit  of  its  orbit  in  365;^  days. 
Then  light  would  make  this  same  circuit  in  y^^ij  of  a  day, 
which  we  find  to  be  52  minutes  8^  seconds.  The  diameter 
of  the  earth's  orbit  is  found  by  dividing  its  circumference  by 
3.1416,  and  the  mean  distance  of  the  sun  is  half  this  diameter. 
We  thus  find  from  the  above  amount  of  aberration  that  light 
passes  from  the  sun  to  the  earth  in  8  minutes  18  seconds. 

The  question  now  arises.  Does  the  same  result  follow  from 
the  observations  of  the  satellites  of  Jupiter?  If  it  does,  we 
have  a  striking  confirmation  of  the  astronomical  theory  of  the 
propagation  of  light.  If  it  does  not,  we  have  a  discrepancy, 
the  cause  of  which  must  be  investigated.  We  have  said  that 
the  first  investigator  of  the  subject  found  the  time  required 
to  be  11  minutes.  This  determination  was,  however,  uncertain 
by  several  minutes,  owing  to  the  very  imperfect  character 
of  the  early  observations  on  which  Koemer  had  to  depend. 
Early  in  the  present  century,  Delambre  made  a  complete  in- 
vestigation from  all  the  eclipses  of  the  satellites  which  had 
been  observed  between  1662  and  1802,  more  than  a  thousand 
in  number.     His  result  Avas  8  minutes  13.2  seconds. 

There  is  a  discrepancy  of  five  seconds  between  this  result 
of  Delambre,  obtained  some  seventy  years  ago,  and  the  mod- 
ern determinations  of  the  aberrations  of  the  fixed  stai*s  made 
by  Struve  and  others.  What  is  its  cause  ?  Probably  oidy  the 
errors  of  the  observations  used  by  Delambre.  .In  this  case, 
there  would  be  no  real  difference.  But  some  physicists  and 
astronomers  have  endeavored  to  show  that  there  is  a  real 
cause  for  such  a  difPerence,  which  they  hold  to  indicate  an  er- 


214  PRACTICAL  ASTRONOMY. 

TOY  in  the  value  of  the  aberration  derived  from  observation 
arising  in  this  way.  It  is  known  from  experiment  that  light 
passes  through  glass  or  any  other  refracting  medium  more 
slowly  than  through  a  void.  In  observations  with  a  telescope 
the  light  has  to  pass  through  the  objective,  and  the  time  lost 
in  doing  so  will  make  the  aberration  appear  larger  than  it 
really  is,  and  the  velocity  of  light  will  appear  too  small.  Jiut 
the  commonly  received  theory  (that  of  Fresnel)  is  that  this 
loss  of  time  is  compensated  by  the  objective  partially  drawing 
the  ray  with  it.  Desirous  of  setting  the  question  at  rest,  Pro- 
fessor Air}',  a  few  years  ago,  constructed  a  telescojje,  which 
he  filled  with  water,  with  which  he  observed  the  constant  of 
aberration.  The  aberration  was  found  to  be  the  same  as  with 
ordinary  telescopes,  thus  proving  the  theory  of  Fresnel  to  be 
correct,  because  on  the  other  theory  the  aberration  ought  to 
have  been  nnicli  increased  by  the  water. 

Hence  this  explanation  of  the  difference  of  the  two  results 
fails,  and  renders  it  more  probable  that  there  is  some  error  in 
Delambre's  result.  A  reinvestigation  of  all  the  observations 
of  Jupiter's  satellites  is  very  desirable  ;  but  so  vast  is  the  labor 
that  no  one  since  Delambre  has  undertaken  it.  Mr.  Glasenapp, 
a  young  Russian  astronomer,  has,  however,  recently  investi- 
gated all  the  observations  of  Jupiter's  first  satellite  made  dur- 
ing the  years  1848-1873,  and  found  from  these  that  the  time 
required  for  light  to  pass  from  the  sun  to  the  earth  is  8  min- 
utes 20  seconds.  Instead  of  being  smaller  than  Struve's  re- 
sult, this  is  two  seconds  larger,  and  seven  seconds  larger  than 
that  of  Delambre.  It  is  therefore  concluded  that  the  differ- 
ence between  the  results  of  the  two  methods  arises  entirely 
from  the  errors  of  the  observations  used  by  Delambre,  and 
that  Struve's  time  (498  seconds)  is  not  a  second  in  error. 

Each  of  the  two  methods  we  have  described  gives  us  the 
time  I'equired  for  light  to  pass  from  the  sun  to  the  earth ;  but 
neither  of  them  gives  us  any  direct  information  respecting  the 
velocity  of  light.  13efore  we  can  determine  the  latter  from 
the  former,  we  must  know  what  the  distance  of  the  sun  is. 
Dividing  this  distance  in  miles  by  498,  we  shall  have  the  dis- 


THE  MOTION   OF  LIGHT.  215 

tance  which  hVht  travels  in  a  seooiid.  Conversely,  if  wo  can 
liiid  expcriincntally  how  far  light  travels  in  a  second,  then  by 
multiply  inii;  this  distance  by  4'JS  wo  shall  have  the  distance  of 
the  sun.  But  we  need  oidy  reflect  that  the  velocity  of  light 
is  about  180,000  miles  per  second  to  see  that  the  problem  of 
determining  it  experimentally  is  a  most  difficult  one.  It  is 
seldom  that  objects  on  the  surface  of  tlie  earth  are  distinctly 
seen  at  a  greater  distance  than  forty  or  fifty  miles,  and  over 
such  a  distance  light  travels  in  the  forty-thousandth  part  of  a 
second.  As  might  be  expected,  the  earlier  attempts  to  fix  the 
time  occupied  by  light  in  passing  over  distances  so  short  as 
those  on  the  surface  of  the  earth  were  entire  failures.  The 
first  of  these  is  due  to  Galileo;  and  his  method  is  worth  men- 
tioning, to  show  the  principle  on  which  such  a  determination 
can  be  made,  lie  stationed  two  observers  a  mile  or  two  apart 
by  night,  each  having  a  lantern  which  he  could  cover  in  a 
moment.  The  one  observer.  A,  was  to  cover  his  lantern,  and 
the  distant  one,  B,  as  soon  as  he  saw  the  light  disappear,  cov- 
ered his  also.  Ill  order  that  A  might  see  the  disappearance 
of  B's  lantern,  it  was  necessary  that  the  light  should  travel 
from  A  to  B,  and  back  again.  For  instance,  if  it  took  one 
second  to  travel  between  the  two  stations,  B  would  continue 
to  see  A's  liglit  an  entire  second  after  it  was  really  extinguish- 
ed ;  and  if  he  then  covered  his  lantern  instantly,  A  would 
still  see  it  during  another  second,  making  two  seconds  in  all 
after  he  had  extinguished  his  own,  besides  the  time  B  might 
have  required  to  completely  perform  the  movement  of  cover- 
ing his. 

Of  course,  by  this  rough  method  Galileo  found  no  inter- 
val whatever.  An  occurrence  which  only  required  the  hun- 
dredth part  of  the  thousandth  of  a  second  was  necessarily  in- 
stantaneous. But  we  can  readily  elaborate  his  idea  into  the 
more  refined  methods  used  in  recent  times.  Its  essential  feat- 
ure is  that  which  must  always  be  employed  in  making  the  de- 
termination ;  that  is,  it  is  necessary  that  the  light  shall  be  sent 
from  one  station  to  another,  and  then  returned  to  the  first 
one,  where  the  double  interval  is  timed.     There  is  no  possi- 


216 


rUACTICAL  ASTRONOMY. 


bility  of  comparing  the  times  at  two  distant  stations  with  the 
necessary  precision.  Tiie  iirst  improvement  we  slioukl  make 
on  Galileo's  method  would  be  to  set  up  a  mirror  at  the  dis- 
tant station,  and  dispense  with  the  second  lantern,  the  ob- 
server A  seeing  his  own  lantern  by  reflection  in  the  mirror. 
Then,  if  he  screened  his  lantern,  he  would  continue  to  see  it 
by  rellection  in  the  mirror  during  the  time  the  light  required 
to  go  and  come.  But  this  also  would  be  a  total  failure,  be- 
cause the  reflection  would  seem  to  vanish  instantly.  Our  next 
effort  would  be  to  try  if  we  could  not  send  out  a  flash  of 
light  from  our  lantern,  and  screen  it  off  before  it  got  back 
again.  An  attempt  to  screen  off  a  single  flash  would  also  be 
a  failure.  We  should  then  try  sending  a  rapid  succession  of 
flashes  through  openings  in  a  moving  screen,  and  see  wheth- 
er they  could  be  cut  off  by  the  sides  of  the  openin:^s  before 

their  return.  This  would  be 
effected  by  the  contrivance 
shown  in  Fig.  60.  We  have 
here  a  wheel  with  spokes  ex- 
tending from  its  circumfer- 
ence, the  distance  between 
them  being  equal  to  their 
breadth.  This  wheel  is  placed 
in  front  of  the  lantern,  L,  so 
that  the  light  from  the  latter 

Fio.  eO.-Uevolvhig  wheel,  for  nieisuring  the    luXS  tO  paSS  betweCll  the  SpokcS 
velocity  of  light.  ^f  jj^^  ^^l^^^j  j,^  ^^.j^^.  ^^  ^.^.^^j, 

the  distant  mirror.  In  the  figure  the  reader  is  snjjposed  to  be 
between  the  wheel  and  the  reflecting  mirror,  facing  the  for- 
mer, so  that  he  sees  the  light  of  the  lantern,  and  also  the  eye 
of  the  observer,  between  the  spokes.  The  latter,  looking  be- 
tween the  spokes,  will  see  the  light  of  the  lantern  reflected 
from  the  mirror.  Now,  suppose  he  turns  the  wheel,  still  keep- 
ing his  eye  at  the  same  point.  Then,  each  spoke  cutting  off  the 
light  of  the  lantern  as  it  passes,  there  will  be  a  succession  of 
flashes  of  light  which  will  pass  through  between  the  spokes, 
travel  to  the  mirror,  and  thence  be  reflected  back  again  to  the 


THE   MOTION  OF  LIGHT.  217 

wheel.  Will  they  reach  the  eye  of  the  observer  behiiul  the 
wheel  ?  Evidently  they  will,  if  they  return  so  quickly  tluit  a 
tooth  has  not  had  time  to  intervene.  But  suppose  the  wheel  to 
turn  so  rapidly  that  a  tooth  just  intervenes  as  the  flash  gets 
back  to  it.  Then  the  observer  will  set  no  light  in  the  mirror, 
because  each  successive  flash  is  caught  by  the  following  tooth 
just  before  it  reaches  the  observer's  eye.  Suppose,  next,  that 
he  doubles  the  speed  of  his  wheel.  Then,  while  the  fla&ii  is 
travelling  to  the  mirror  and  back,  the  tooth  will  liave  passed 
clear  across  and  out  of  the  way  of  the  flash,  so  that  i  latt^M* 
will  now  reach  the  observer's  eye  through  the  opening  next 
following  that  which  it  passed  through  to  leave  the  lantern. 
Thus,  the  observer  will  see  a  succession  of  flashes  so  rapid 
that  they  will  seem  entirely  continuous  to  the  eye.  If  the 
speed  of  the  wheel  be  again  increased,  the  return  flash  will  be 
caught  on  the  second  tooth,  and  the  observer  will  see  no  light, 
while  a  still  further  increase  of  velocity  will  enable  him  to 
see  the  flashes  as  they  return  through  the  second  interval  be- 
tween the  spokes,  and  so  on. 

In  principle,  this  is  Fizeau's  method  of  measuring  the  ve- 
locity of  light.  In  place  of  spokes,  he  has  exceedingly  flne 
teeth  in  a  larije  wheel.  lie  does  not  look  between  the  teeth 
with  the  naked  eye,  but  employs  a  telescope  so  arranged  that 
the  teeth  pass  exactly  through  its  focus.  An  arrangement  is 
made  by  which  the  light  passes  through  the  same  focus  with- 
out reaching  the  observer's  eye  except  by  reflection  from  the 
distant  iuirror.  The  latter  is  placed  in  the  focus  of  a  second 
telescope,  so  that  it  can  be  easily  adjusted  to  send  the  rays 
back  in  the  exact  direction  from  which  they  come.  To  And 
the  time  it  takes  the  light  to  travel,  it  is  necessary  to  know  the 
exact  velocity  of  the  wheel  which  will  cut  off  the  return  light 
ntirely,  and  thence  the  number  of  teeth  which  pass  in  a  sec- 
ond. Suppose,  for  instance,  that  the  wheel  had  a  thousand 
teeth,  and  the  reflector  was  nine  miles  aw'ay,  so  that  the  light 
had  to  travel  eighteen  miles  to  get  back  to  the  focus  of  the 
telescope.  Then  it  would  be  found  that  with  a  velocity  of 
about  five  turns  of  the  wheel  per  second,  the  light  would  be 


c 


218  PRACTICAL  ASTRONOMY. 

first  cut  off.  Increasing  the  velocity,  it  would  reappear,  and 
would  grow  brighter  until  the  velocity  reached  ten  turns  per 
second.  It  would  then  begin  to  fade  away,  and  at  lifteen 
turns  per  second  would  be  again  occulted,  and  so  on.  With 
the  latter  velocity,  fifteen  thousand  teetli  and  fifteen  thousand 
intervals  would  pass  in  a  second,  while  two  teeth  and  one  in- 
terval passed  during  the  time  the  light  was  performing  its 
journey.  Tiie  latter  would,  therefore,  be  performed  in  the 
ten-thousandth  part  of  a  second,  showing  the  actual  velocity 
to  be  180,000  miles  per  second.  The  most  recent  determina- 
tion made  in  this  way  is  by  M.  Cornu,of  Paris,  who  has  made 
some  improvements  in  the  mode  of  applying  it.  His  results 
will  be  described  ]>resently. 

Ingenious  and  beautiful  as  this  method  is,  I  do  not  think  it 
can  be  so  accurate  as  another  employed  by  Foucault,  in  wliich 
it  is  not  a  toothed  wheel  which  revolves,  but  a  AVheatstone 
mirror.    To  explain  the  details  of  the  apparatus  actually  used 

would    be    tedious, 

,^  but  the  principle  on 

\\  which  the   method 

\\  rests    can    be   seen 

--^A-S"       quite  readily.    Sup- 

^^''     \       pose  A  B,  Fig.  61,  to 

^  ^^/'  \\^'  represent  a  fiat  mi r- 

y^o  /  I'OJ'i  seen  edgewise, 

revolving  round  an 
jp^y  axis  at  X,  and  C  a 

Fio.  61.— Illustrating  Foucnult's  method  of  measuring  the   fixed    COUCaVG    mir- 
velocity  of  lij^ht.  ^^^^  g^    ^^^^^^^^    ^^^^^ 

the  centre  of  its  concavity  shall  fall  on  A".  Let  0  be  a  lumi- 
nous point,  from  which  emanates  a  single  ray  of  light,  OA'. 
This  ray,  meeting  the  mirror  at  A',  is  reflected  to  the  concave 
mirror,  C,  wliich  it  meets  at  a  right  angle,  and  is  therefore  re- 
flected directly  back  on  the  line  from  which  it  came,  first  to 
A',  and  then  through  the  point  0,  from  which  it  emanated,  so 
that  an  eye  stationed  nt  I'J  will  see  it  returning  exactly  through 
the  point  0.     No  matter  how  the  observer  may  turn  the  mir- 


THE  MOTION  OF  LIGHT.  219 

ror  AB,  he  cannot  make  the  reflected  ray  deviate  from  this 
line :  he  can  only  make  it  strike  a  different  point  of  the  mir- 
ror C.  If  he  turns  AB  so  that  after  the  ray  is  reflected  from 
it,  it  does  not  strike  C  at  all,  then  he  will  see  no  return  ray. 
If  the  ray  is  reflected  back  at  all,  it  will  pass  through  0.  Tiiis 
result  is  founded  on  the  supposition  that  the  mirror  AB  re- 
mains in  the  same  position  during  the  time  the  ray  occupies 
in  passing  from  X  to  C  and  back.  But  snppose  the  mirror 
AB  to  be  revolving  so  rapidly  that  when  the  ray  gets  back 
to  X,  the  mirror  has  moved  to  the  position  of  the  dotted  line 
A'B'.  Then  it  will  no  longer  be  reflected  back  through  0, 
but  will  be  sent  in  the  direction  17,  the  angle  EXE'  being 
double  that  through  which  the  mirror  has  moved  during  the 
time  the  ray  was  on  its  passage.  Knowing  tlio  velocity  of 
the  mirror,  and  the  angle  EXE',  this  time  is  easily  found. 

Evidently  the  observer  cannot  see  a  continuous  light  at  E', 
because  a  reflection  can  be  sent  back  only  when  the  revolving 
mirror  is  in  such  a  position  as  to  send  the  ray  to  some  point 
of  the  concave  mirror,  C!  What  will  really  be  seen,  therefore, 
is  a  succession  of  flashes,  each  flash  appearing  as  the  revolving 
mirror  is  passing  througli  the  position  AB.  But  when  the 
mirror  revolves  rapidly,  these  flashes  will  seem  to  the  eye  to 
form  a  continuous  light,  which,  however,  will  be  fainter  than 
if  the  mirror  were  at  rest,  in  the  proportion  which  the  arc  of 
the  concave  mirror,  C,  bears  to  an  entire  circle.  Beyond  the 
enfeeblemcnt  of  the  light,  this  want  of  continuity  is  not  pro- 
ductive of  any  inconvenience.  It  was  thus  found  by  Fou- 
cault  that  the  velocity  of  light  was  185,000  miles  per  second,  a 
result  which  is  probably  within  a  thousand  miles  of  the  truth. 

The  preceding  explanation  shows  the  principle  of  the  meth- 
od, but  not  the  details  necessary  in  applying  it.  It  is  not 
practicable  to  isolate  a  single  ray  of  light  in  the  manner  sup- 
posed in  the  figure,  and  therefore,  without  other  apparatus, 
the  light  from  0  would  be  spread  all  over  the  space  around  E 
and  E'.  The  desired  result  is  obtained  by  placing  a  lens  be- 
tween the  luminous  point  0  and  the  revolving  mirror  in  such 
a  position  that  all  the  light  falling  from  0  upon  the  lens  shall, 


220  PliACTICAL  ASTRONOMY. 

after  reflection,  be  brought  to  a  focus  upon  the  surface  of  the 
concave  mirror,  C.  Tlien  when  the  mirror  AB  is  made  to  re- 
volve rapidly,  the  return  rays  passing  back  through  the  lens 
on  their  return  journ».y  are  brought  to  a  focus  at  a  point 
along-side  0,  and  distant  from  it  by  an  amount  which  is  pro- 
portional to  the  time  the  light  has  required  to  pass  from  X  to 
Cand  back  again. 

So  delicate  is  this  method,  that  the  millionth  of  a  second  of 
time  can  be  measured  by  it  as  accurately  as  a  carpenter  can 
measure  the  breadth  of  a  board  with  liis  rule.  Its  perfection 
is  the  result  of  the  combined  genius  of  several  men.  The  first 
idea  of  employing  a  revolving  mirror  in  tlie  measurement  of 
a  very  minute  interval  of  time  is  due  to  the  late  Sir  Charles 
Wheatstone,  wlio  thus  measured  the  duration  of  the  electric 
spark.  Then  Arago  showed  that  it  could  be  applied  to  de- 
termine whether  the  velocity  of  light  was  greater  in  water 
or  in  air.  Fizeau  and  Foucault  improved  on  Arago's  ideas 
by  the  introduction  of  the  concave  mirror,  having  its  centre 
of  curvature  in  the  revolving  mirror,  and  then  this  wonderful 
piece  of  apparatus  was  substantially  complete.  The  last  de- 
termination of  the  velocity  of  light  witli  it  was  made  b}'  Fou- 
cault, and  communicated  to  the  French  Academy  of  Sciences 
in  18G2,  with  the  statement  that  the  velocity  resulting  from 
all  his  experiments  was  298,000  kilometres  (185,200  miles) 
per  second. 

The  problem  in  question  was  next  taken  up  by  Cornu,  of 
Paris,  whose  result  lias  already  been  alluded  to.  Kotwith- 
standing  the  supposed  advantages  of  the  Foucault -Wheat- 
stone  method,  M.  Cornu  preferred  that  of  Fizeau.  His  first 
results,  reached  in  1872,  accorded  quite  well  with  those  of 
Foucault  just  cited,  indicating  a  small  but  somewhat  uncer- 
tain increase.  His  experiments  were  repeated  in  1874,  and 
their  results  were  communicated  to  the  French  Academy  of 
Sciences  in  December  of  that  year.  In  this  last  series  of 
measurements  his  station  was  the  observatory,  .and  the  distant 
mirror  was  placed  on  the  tower  of  Montliiery,  at  a  distance  of 
about  fourteen  English  miles.     The  telescope  through  which 


THE  MOTION  OF  LIGHT. 


221 


the  flashes  of  hght  were  sent  and  received  was  twenty-nine 
feet  long  and  of  fourteen  inches  rperture.  The  velocity  of 
the  toothed  wheel  could  be  made  to  exceed  1600  turns  a  sec- 
ond, and  by  the  electro-chronograph,  on  which  the  revolutions 
were  recorded,  the  time  could  be  determined  within  the  thou- 
sandth of  a  second.  At  Montlhery,  the  telescope,  in  the  focus 
of  which  the  reflecting  mirror  was  placed,  was  six  inches  in 
aperture,  and  was  held  by  a  large  cast-iron  tube  set  in  the 
masonry  of  the  tower.  At  this  distance  M.  Cornu  was  able, 
with  the  highest  velocity  of  his  revolving  wheel,  to  make 
twenty  of  its  teeth  pass  before  the  flashes  of  light  got  back, 
and  to  catch  them,  on  their  retnrn,  on  the  twenty-first  tooth. 

All  the  determinations,  however,  were  not  made  with  the 
wheel  going  at  this  rate,  but  with  such  different  velocities  that 
the  rays  were  caught  sometimes  on  one  tooth  and  sometimes 
on  another,  from  the  fourth  to  the  twenty-first.  The  follow- 
ing table  shows  the  velocity  of  light  in  kilometres  per  second 
when  the  ray  was  caught  on  the  fourth  tooth,  on  the  fifth,  and 
so  on  to  the  twenty-first : 


Tooth  4 800,130 

"     5 3oo,r);u) 

"      G 300,7r)0 

"       7 300,820 

"      8 2i);),!)40 

"    !) 3oo,r)r)0 

"    10 300,040 

"    11 3OO,3ri0 

"    12 300,r)00 


Tooth  13 300,340 

"      14 300,350 

"      15 300,21)0 

"      10 300,020 

"      17 300,000 

"      18 300,150 

"      19 201»,55O 

"      20 

"      21 300,000 


M.  Cornu  hence  concludes  that  the  velocity  of  light  in  air 
is  300,-330,  and  in  a  vacuum  300,400  kilometres  per  second. 
But  Ilelmert,  of  Aix,  has  noticed  a  tendency  in  M.  Cornu's 
numbers,  as  given  above,  to  diminish  as  the  velocity  of  the 
wheel  is  increased,  and  concludes  that  the  true  velocity  to  be 
derived  from  the  measures  is  299,900  kilometres.  This  re- 
sult, thougli  less  than  that  derived  by  Cornu  himself,  is  still 
nearly  2000  kilometres  greater  than  that  of  Foucault. 


222  FRAVTICAL  ASTRONOMY. 


CHAPTER  Y. 

THE     SPECTROSCOPE. 

In  one  of  Dr,  Lardner's  popular  lectures  on  astronomy,  de- 
livered some  thirty  years  ago,  lie  introduced  the  subject  of 
weighing  the  planets  as  one  in  which  he  could  with  difficulty 
expect  his  statements  to  be  received  with  credulity.  That 
men  should  measure  the  distances  of  the  planets  was  a  state- 
ment he  expected  his  hearers  to  receive  with  surprise ;  but  the 
step  from  measuring  to  weighing  was  so  long  a  one,  that  it 
seemed  to  the  ordinary  mind  to  extend  beyond  all  the  bounds 
of  possibility. 

Had  a  hearer  told  the  lecturer  that  men  would  also  be  able 
to  determine  the  chemical  constituents  of  the  sun  and  stars, 
and  to  tell  whether  any  of  them  did  or  did  not  contain  iron, 
hydrogen,  and  other  chemical  elements,  the  lecturer  would 
probably  have  replied  that  that  statement  quite  exceeded  the 
limits  of  his  own  credulity;  that,  while  he  himself  saw  clearly 
how  the  planets  -were  measured  and  M'eighed,  he  looked  upon 
the  idea  of  determining  their  chemical  constitution  as  a  mere 
piece  of  pleasantry,  or  the  play  of  an  exuberant  fancy.  And 
yet,  this  very  thing  has,  to  a  certain  extent,  been  done  by  the 
aid  of  the  spectroscope.  The  chemical  constitution  of  matter 
in  the  state  of  gas  or  vapor  can  be  detected  almost  as  readily 
at  the  distance  of  the  stars  as  if  we  had  it  in  our  laboratories. 
The  difficulties  which  stand  in  the  way  do  not  arise  from  the 
distance,  but  from  the  fact  that  matter  in  the  heavenly  bodies 
seems  to  exist  in  some  state  which  we  have  not  succeeded  in 
exactly  reproducing  in  our  laboratories.  Like  many  other 
wonders,  spectrum  analysis,  as  it  is  called,  is  not  at  all  extraor- 
dinary after  we  sec  how  it  is  done.     Indeed,  the  only  wonder 


THE  SPECTROSCOPE.  223 

now  is  how  the  first  half  of  this  century  could  have  passed 
without  physicists  discovering  it.  The  essential  features  of 
the  method  are  so  simple  that  only  a  knowledge  of  the  ele- 
ments of  natural  philosophy  is  necessary  to  enable  them  to  be 
understood.     We  shail,  therefore,  briefly  explain  them. 

It  is  familiarly  known  that  if  we  pass  the  rays  of  the  sun 
which  enter  a  room  by  a  small  opening  through  a  prism,  the 
light  is  separated  into  a  number  of  bright  colors,  which  are 
spread  out  on  a  certain  scale,  the  one  end  being  red  and  the 
other  violet,  while  a  long  range  of  intermediate  colors  is  found 
between  them.  This  shows  that  common  white  light  is  really 
a  compound  of  every  color  of  the  spectrum.  This  compound 
is  not  like  chemical  compounds,  made  up  of  two  or  three  or 
some  limited  number  of  simples,  but  is  composed  of  an  infini- 
ty of  different  kinds  of  light,  all  running  into  eacli  other  by 
insensible  degrees ;  the  difference,  however,  being  only  in  col- 
or, or  in  the  capacity  of  being  refracted  by  the  prism  through 
which  it  passes.  This  arrangement  of  colors,  spread  out  to  our 
sight  according  to  the  refrangibility  of  the  light  M'liich  forms 
them,  is  called  the  spectrum.  By  the  spectrum  of  any  object 
is  meant  the  combination  of  colors  found  in  the  light  which 
emanates  from  that  object.  For  instance,  if  we  pass  the  light 
from  a  candle  through  a  prism,  so  as  to  separate  it  into  its 
cowiponcnt  colors,  and  make- the  light  thus  separated  fall  on 
a  screen,  the  arrangement  of  colors  on  the  screen  would  be 
called  the  spectrum  of  the  candle.  If  we  look  at  a  bright 
star  through  a  prism,  the  combination  of  colors  which  we  see 
is  called  the  spectrum  of  the  star,  and  so  with  any  other  object 
we  may  choose  to  examine. 

As  the  experiment  of  forming  a  spectrum  is  commonly 
made,  there  is  a  slight  mixing-up  of  light  of  the  different  col- 
ors, because  light  of  the  same  degree  of  refrangibility  will 
fall  on  different  parts  of  the  screen  according  to  the  ]iart  of 
the  prism  it  passes  through.  Wlien  the  separation  of  the  light 
is  thus  incomplete,  the  spectrum  is  said  to  be  impure.  In  or- 
der to  make  anv  successful  examination  of  the  light  which 
emanates  from  ai.  object,  our  spectrum  must  be  pure;  that  is, 


224  riiACTICAL  ASTRONOMY. 

each  point  of  the  spectrum  must  be  formed  by  light  of  one 
degree  of  refrangibility.  To  effect  this  in  the  most  perfect 
way,  the  spectrum  is  not  formed  on  a  screen,  but  on  the  retina 
of  the  observer's  eye.  An  instrument  by  which  this  is  done 
is  called  a  spectroscope. 

The  most  essential  parts  of  a  spectroscope  consist  of  a  small 
telescope  with  a  prism  in  front  of  the  object-glass.  The  ob- 
server must  adjust  his  telescope  so  that,  removing  the  prism, 
and  looking  directly  at  the  object,  he  shall  obtain  distinct  vis- 
ion of  it.  Then,  putting  the  prism  in  its  place,  and  turning 
the  telescope  to  such  an  angle  that  the  light  which  comes  from 
the  object  shall,  after  being  refracted  by  the  prism,  pass  direct- 
ly into  the  telescope,  he  looks  into  the  latter.  When  the  prop- 
er adjustments  are  made,  he  will  see  a  pure  spectrum  of  the 
object.  In  order  that  this  experiment  may  succeed,  it  is  es- 
sential that  the  object,  when  viewed  directly,  shall  present  the 
appearance  of  a  point,  like  a  star  or  planet.  If  it  is  an  object 
which  has  a  measurable  surface,  like  the  sun  or  moon,  he  will 
see  either  no  spectrum  at  all  or  only  a  very  impure  one. 

For  this  reason,  a  spectroscope  which  consists  of  nothing  but 
a  telescope  and  prism  is  not  fitted  for  any  purpose  but  that  of 
trial  and  illustration.  To  tit  it  for  general  use,  another  ob- 
ject-glass, with  a  slit  in  its  focus,  is  added.     Fig.  G2  shows  the 


Fig,  62.— Course  of  rnys  through  a  Bpectroscope. 

essential  parts  of  a  modern  spectrosco]>e.  At  the  farther  end 
of  the  second  telescope,  where  the  light  enters,  is  a  narrow 
slit,  which  can  be  opened  or  closed  by  means  of  a  screw,  and 


THE  SPECTROSCOPE.  225 

tliroiigli  which  the  light  from  the  object  is  admitted.  The 
rays  of  light  following  the  dotted  lines  are  made  parallel  by 
passing  through  the  lens,  L,  They  then  fall  on  the  prism,  P, 
by  which  they  are  refracted,  and  from  which  they  emerge  par- 
allel, except  that  the  direction  of  the  rays  of  different  colors 
is  different,  owing  to  the  greater  or  less  degree  of  refraction 
produced  by  the  prism.  They  then  pass  through  the  object- 
glass  of  the  telescope,  T,  by  which  the  rays  of  each  color  are 
brought  to  a  focus  at  a  particular  point  in  the  Held  of  view, 
the  red  rays  all  coming  together  at  the  lower  point,  the  violet 
ones  at  the  upper  point,  and  those  of  each  intermediate  color 
at  their  proper  place  along  the  line.  The  observer,  looking 
into  the  telescope,  sees  the  spectrum  of  whatever  object  is 
throwing  its  light  through  the  slit. 

If  the  object  of  which  the  observer  wishes  to  see  the  speo- 
trum  is  a  flame,  he  places  it  immediately  in  front  of  the  slit; 
and  if  it  is  an  object  of  sensible  surface,  like  the  sun  or  moon, 
he  points  the  collimator,  C,  directly  at  it,  so  that  the  light 
which  enters  the  slit  shall  fall  on  the  lens,  Z.  But  if  it  is  a 
star,  lie  cannot  get  light  enough  in  this  way  to  see  it,  and  he 
must  either  remove  his  collimator  entirely,  or  fasten  his  spec- 
troscope to  the  end  of  a  telescope,  so  that  tb;^  slit  shall  be 
exactly  in  the  foci'ts.  Tfte*  latter  is  the  method  universally 
adopted  in  examining  the  spectrum  of  a  star. 

If,  with  this  instrument,  we  examine  the  light  which  comes 
from  a  candle,  from  the  fire,  or  from  a  piece  of  white-hot 
iron,  we  shall  find  it  to  be  continuous ;  that  is,  there  is  no  gap 
in  the  series  of  colors  from  one  end  to  the  other.  But  if  we 
take  the  light  from  the  sun,  or  from  the  moon,  a  planet,  or 
any  object  illuminated  by  the  sun,  we  shall  find  the  spectrum 
to  be  crossed  by  a  great  number  of  fine  dark  lines,  showing 
that  certain  kinds  of  light  are  wanting.  It  is  now  known 
that  tlie  particular  kinds  of  light  which  originally  belonged 
in  these  dark  lines  have  been  culled  out  by  the  gases  surround- 
ing the  sun  through  which  the  light  has  passed.  This  culling- 
ont  is  called  ^Selective  Absorption.  It  is  found  by  experiment 
that  each  kind  of  gas  has  its  own  liking  for  light  of  peculiar 

16 


226  PRACTICAL   ASTRONOMY. 

degrees  of  refrangibility,  and  absorbs  the  light  which  belongs 
in  the  corresponding  parts  of  the  spectrum,  letting  all  the 
other  light  pass. 

Perhaps  we  may  illustrate  this  process  by  a  similar  one 
which  we  might  imagine  mankind  to  perform.  Suppose  Nat- 
ure should  loan  us  an  immense  collection  of  many  millions 
of  gold  pieces,  out  of  which  we  were  to  select  those  which 
would  serve  us  for  money,  and  return  her  the  remainder. 
The  English  rummage  through  the  pile,  and  pick  out  all  the 
pieces  which  are  of  the  proper  weight  for  sovereigns  and  half- 
sovereigns  ;  the  French  pick  out  those  which  will  nuike  five, 
ten,  twenty,  or  fifty  franc  pieces ;  the  Americans  the  one,  live, 
ten,  and  twenty  dollar  pieces,  and  so  on.  After  all  the  suit- 
able pieces  are  thus  selected,  let  the  remaining  mass  be  spread 
out  on  the  ground  according  to  the  respective  weights  of  the 
pieces,  the  smallest  pieces  being  placed  in  a  row,  the  next  in 
weight  in  an  adjoining  row,  and  so  on.  We  shall  then  find  a 
number  of  rows  missing :  one  w^hich  the  French  have  taken 
out  for  five-franc  pieces;  close  to  it  another  which  the  Amer- 
icans have  taken  for  dollars;  afterwards  a  row  which  have 
gone  for  half-sovereigns,  and  so  on.  By  thus  arranging  the 
pieces,  one  would  be  able  to  tell  what  nations  had  culled  over 
the  pile,  if  he  only  knew  of  what  weight  each  one  made  its 
coins.  The  gaps  in  the  places  where  the  sovereigns  and  half- 
sovereigns  belonged  would  indicate  the  English,  that  in  the 
dollars  and  eagles  the  Americans,  and  so  on.  If,  now,  we  re- 
flect how  utterly  hopeless  it  would  appear,  from  the  mere  ex- 
amination of  the  miscellaneous  pile  of  pieces  which  had  been 
left,  to  ascertain  what  people  had  been  selecting  coins  from  it, 
and  how  easy  the  problem  would  appear  when  once  some 
genius  should  make  the  proposed  arrangement  of  the  pieces 
in  rows,  we  shall  see  in  what  the  fundamental  idea  of  spec- 
trum analysis  consists.  The  formation  of  the  spectrum  is  the 
separation  and  arrangement  of  the  light  w^hich  comes  from  an 
object  on  the  same  system  b}'^  which  we  have  supposed  the 
gold  pieces  to  be  arranged.  The  gaps  we  see  in  the  spectrum 
tell  the  tale  of  the  atmosphere  tln-ough  which  the  light  has 


THE  SPECTROSCOPE.  227 

passed,  as  in  the  case  of  the  coins  they  would  tell  what  nations 
had  sorted     .  er  the  pile. 

That  the  dark  lines  in  the  solar  spectrum  are  picked  out  by 
the  gases  of  the  sun's  atmosphere  has  long  been  surmised  ;  in- 
deed, Sir  John  Ilerschel  seems  to  have  had  a  clear  idea  of 
the  possibility  of  spectrum  analysis  half  a  century  ago.  The 
difficulty  was  to  find  what  particular  lines  any  particular  sub- 
stance selects;  since,  to  exert  any  selective  action,  a  vastly 
greater  thickness  of  gas  is  generally  required  than  it  is  prac- 
ticable to  obtain  experimentally.  This  difficulty  was  sur- 
mounted by  the  capital  discovery  of  Kirchhoff  and  Bunsen, 
that  a  (jloivimj  gas  gives  out  rays  of  the  same  degree  of  refrangibil- 
ity  ivhich  it  absorbs  when  light  passes  through  it.  For  example, 
if  we  put  some  salt  into  the  flame  of  a  spirit-lamp,  and  ex- 
amine the  spectrnin  of  the  light,  we  shall  find  a  pair  of  bright- 
yellow  lines,  which  correspond  most  accurately  to  a  pair  of 
black  lines  in  the  solar  spectrum.  These  lines  are  known  to 
be  due  to  sodium,  a  component  of  common  salt,  and  their  ex- 
istence in  the  solar  spectrum  shows  that  there  is  sodium 
in  the  sun's  atmosphere.  They  are  therefore  called  the  sodi- 
um lines.  By  vaporizing  various  substances  in  sufficiently  hot 
flames,  the  spectra  of  a  great  number  of  metals  and  gases 
have  been  found.  Sometimes  there  are  only  one  or  two  bright 
lines,  while  with  iron  the  number  is  counted  by  hundreds. 
The  quantity  of  a  substance  necessary  to  form  these  bright 
lines  is  so  minute  tiiat  tlie  presence  of  some  metals  in  a  com- 
pound have  been  detected  with  the  spectroscope  when  it  was 
impossible  to  find  a  trace  of  them  in  any  other  way.  Indeed, 
two  or  three  new  metals,  the  existence  of  which  was  before  en- 
tirely unknown,  first  told  their  story  through  the  spectroscope. 

The  general  relations  of  the  spectrum  to  the  state  of  the 
substance  from  which  the  light  emanated  may  be  conaensed 
into  three  rules,  or  laws,  as  follows : 

1.  The  light  from  a  glowing  solid  or  liquid  forms  a  contin- 
uous spectrum,  in  which  neither  bright  nor  dark  lines  are 
found.  The  spectrum  is  of  the  same  nature,  no  matter  liow 
finely  the  substance  may  be  divided. 


228  PRACTICAL  ASTRONOMY. 

2.  If  the  light  from  the  glowing  solid  passes  through  a  gas- 
eous atmosphere,  the  spectrum  will  be  crossed  by  dark  lines 
occupying  those  parts  of  the  spectrum  where  the  light  culled 
out  by  the  atmosphere  belongs. 

3.  A  glowing  gas  sends  out  light  of  the  same  degrees  of 
refrangibility  as  belong  to  that  which  it  absorbs,  so  that  its 
spectrum  consists  of  a  system  of  bright  lines  occupying  the 
same  position  as  the  dark  lines  it  would  produce  by  absorption. 

If,  then,  on  examining  the  spectrum  of  a  star  or  other  heav- 
enly body,  we  find  only  bright  lines  with  dark  spaces  between 
them,  we  may  conclude  that  the  body  consists  of  a  glowing 
gas,  and  we  judge  what  the  gas  is  by  comparing  the  spectrum 
with  those  of  various  substances  on  the  earth.  If,  on  the  oth- 
er hand,  the  spectrum  is  a  continuous  one,  except  where  cross- 
ed by  fine  dark  lines,  we  conclude  that  it  emanates  from  a 
glowing  body  surrounded  by  an  atmosphere  which  culls  out 
some  of  the  rays  of  light. 

It  will  be  seen  that  the  spectroscope  gives  us  no  definite  in- 
formation respecting  the  nature  or  composition  of  bodies  in 
the  solid  state.  If  we  heat  any  sort  of  metal  white-hot,  sup- 
posing only  that  it  will  stand  this  heat  without  being  vapor- 
ized, we  shall  have  a  spectrum  continuous  from  end  to  end,  in 
which  there  will  be  neither  brignt  nor  dark  lines  to  give  any 
indications  respecting  the  substance.  In  order,  therefore,  to 
detect  the  presence  of  any  chemical  element  with  this  instru- 
ment, that  element  must  be  in  the  form  of  gas  or  vapor.  Here 
we  have  one  limitation  to  the  application  of  the  spectroscope 
to  the  celestial  bodies.  The  tendency  of  bodies  in  space  is  to 
cool  off,  and  when  they  have  once  become  so  cool  as  to  solidi- 
fy, the  instrument  in  question  can  give  us  no  further  definite 
information  respecting  their  constitution. 

Even  if  the  body  be  in  the  gaseous  state,  we  cannot  always 
rely  on  the  spectroscope  informing  us  with  certainty  of  the 
nature  of  the  gas.  The  light  we  analyze  must  either  be  emit- 
ted by  the  gas,  the  latter  being  so  hot  as  to  shine  by  its  own 
light,  or  it  must  be  transmitted  through  it.  Thus,  the  appli- 
cation of  spectrum  analysis  is  confined  to  glowing  gases  and 


THE  SPECTROSCOPE.  229 

the  atmospheres  of  the  stars  and  planets,  the  application  to  the 
latter  depending  on  the  fact  that  the  sunlight  reflected  from 
the  surface  of  the  i)lanet  passes  twice  through  its  atmosphere. 
Even  in  these  cases  the  interpretation  of  its  results  is  sometimes 
rendered  difHcult  in  consecpience  of  the  varied  spectrum  of  the 
same  gas  at  diiferent  temperatures  and  under  different  degrees 
of  pressure.  Under  some  conditions  so  many  new  lines  are 
introduced  into  the  spectrum  of  hydrogen  that  it  can  hardly 
be  recognized.  As  a  general  rule,  the  greater  the  pressure,  the 
greater  the  number  of  lines  which  ai)pear ;  indeed,  it  has  been 
found  by  Lockyer  and  Frankland  that  as  the  pressure  and  den- 
sity of  a  gas  are  increased,  its  spectrum  tends  to  become  con- 
tinuous. We  must  therefore  regard  the  third  of  the  above 
rules  respecting  spectrum  analysis,  or,  rather,  the  general  rule 
that  a  glowing  gas  gives  a  spectrum  of  bright  lines,  as  not  uni- 
versally t'*ue.  If  we  could,  by  artiflcially  varying  the  temper- 
ature, pre  ,sure,  and  composition  of  gases,  accurately  reproduce 
the  spectrum  of  a  celestial  body,  the  changes  of  the  spectrum 
which  \VG  liave  mentioned  would  be  a  positive  advantage; 
since  they  would  enable  us  to  determine,  not  merely  the  com- 
position of  a  gaseous  body,  but  its  temperature  and  pressure. 
This  is,  however,  a  field  in  which  success  has  not  yet  been 
reached. 

The  reader  now  understands  that  when  the  light  from  a  ce- 
lestial object  is  analyzed  by  the  prism,  and  the  component  col- 
ors are  spread  out  singly  as  on  a  sheet,  the  dark  and  bright 
lines  which  we  see  are  the  letters  of  the  open  book  which  we 
are  to  interpret  so  as  to  learn  what  they  tell  us  of  the  body 
from  which  the  light  came,  or  the  vapors  through  which  it 
passed.  When  we  see  a  line  or  a  set  of  lines  which  we  rec- 
ognize as  produced  by  a  known  substance,  we  infer  the  pres- 
ence of  that  substance.  The  question  may  now  be  asked.  How 
do  we  know  but  that  the  lines  we  observe  may  be  prod 'iced 
by  other  substances  besides  those  which  we  find  to  produce 
them  in  our  laboratories  ?  May  not  the  same  lines  be  pro- 
duced b}'^  different  substances?  This  question  can  be  an- 
swered only  by  an  appeal  to  probabilities.     The  evidence  in 


230  PRACTICAL  ASTRONOMY. 

tlie  case  is  mncli  the  same  as  that  by  which,  recognizing  the 
picture  of  a  friend,  we  conchide  that  it  is  not  the  picture  of 
any  one  else.  For  anything  we  can  prove  to  tlie  contrary, 
anotlier  person  might  have  exactly  the  same  features,  and 
nn"glit,  therefore,  make  tlie  very  same  picture.  But,  as  a  mat- 
ter of  fact,  we  know  that  practically  no  tv/o  men  whom  we 
have  ever  seen  do  look  exactly  alike,  and  it  is  extremely  im- 
probable that  they  ever  would  look  so.  The  case  is  the  same 
in  spectrum  analysis.  Among  the  great  number  of  substances 
which  have  been  examined  v;itli  the  spectroscope,  no  two  give 
the  same  lines.  It  is  therefore  extremely  improbable  that  a 
given  system  of  bright  lines  could  be  produced  by  more  than 
one  substance.  At  the  same  time,  the  evidence  of  the  spec- 
troscope is  not  necessarily  conclusive  in  all  cases.  Should 
only  a  single  line  of  a  substance  be  found  in  the  spectrum  of 
a  star  or  nebula,  it  would  hardly  be  safe  to  conclude,  from  that 
alone,  that  the  linr  was  really  produced  by  the  known  sub- 
stance. Collateral  evidence  might,  however,  come  in.  If  the 
same  line  were  found  both  in  the  sunlight,  and  in  that  of  a 
great  number  of  stars,  we  should  be  justified  in  concluding 
that  the  lines  were  all  produced  by  the  same  substance.  All 
we  can  say  in  doubtful  cases  is,  that  our  conclusions  must  be 
drawn  with  care  and  discrimination,  and  must  accord  with  the 
probabilities  of  each  special  case. 


PART  III.  — THE  SOLAR  SYSTEM. 


CHAPTER  I. 

GENERAL    STRUCTURE   OF    THE    SOLAR    SYSTEM. 

Having,  in  the  preceding  parts,  described  the  general  struct- 
ure of  the  universe,  and  the  nietliods  used  by  astronomers  in 
measuring  tlie  heavens  and  investigating  the  celestial  motions, 
we  have  next  to  consider  in  detail  the  separate  bodies  which 
compose  the  universe,  and  to  trace  the  conclusions  respecting 
the  general  order  of  creation  to  which  this  examination  may 
lead  us.  Our  natural  course  will  be  to  begin  with  a  general 
description  of  the  solar  system  to  which  our  earth  belongs, 
considering,  first,  the  great  central  body  of  tliat  system,  then 
the  planets  in  their  order,  and,  lastly,  such  irregular  bodies  as 
comets  and  meteors. 

We  have  shown  in  the  fii*st  part  that  the  solar  system  was 
found  by  Copernicus,  Kepler,  and  Newton  to  consist  of  the 
sun,  as  the  great  central  body,  with  a  number  of  planets  re- 
volving around  it  in  ellipses,  having  the  sun  in  one  of  their 
foci ;  the  whole  being  bound  together  In'  the  law  of  univereal 
gravitation.  Modern  science  has  added  a  great  number  of 
bodies,  and  shown  the  system  to  be  a  much  more  complex  one 
tlian  Newton  supposed.  As  we  now  know  them,  the  bodies 
of  the  system  may  be  classified  as  follows : 

1.  The  sun,  the  great  central  body ; 

2.  A  group  of  four  inner  planets  —  Mercury,  Venus,  the 
Earth,  and  Mars ; 

3.  A  swarm  of  small  planets  or  asteroids  revolving  outside 
the  orbit  of  Mars  (about  175  of  them  are  now  known) ; 


232  THE  SOLAR  SYSTEM. 

4.  A  group  of  four  outer  planets — Jupiter,  Saturn,  Uranus, 
and  Neptune ; 

5.  A  number  of  satellites  of  the  planets,  18  being  now 
known,  of  which  all  but  one  belong  to  the  group  of  outer 
planets ; 


Fi<).  C;!.  — IJehitivo  si/.o  of  t-iiii  and  planets. 

C.  An  unknown  number  of  comets  and  meteors,  revolving 
in  very  eccentric  orbits. 

The  eight  planets  of  groups  2  and  4  are  called  the  major 
planets,  to  distinguish  them  from  all  others,  which  are  smaller 
or  less  important. 


GENERAL  STRUCTURE  OF  THE  SOLAR  SYSTEM.        233 


The  range  of  size,  distance,  and  mass  among  the  bodies  of 
tlie  system  is  enormous.  Neptune  is  eighty  times  as  far  from 
tlie  sun  as  Mercury,  and  Jupiter  several  thousand  times  as 
heavy.  It  is,  therefore,  difficult  to  lay  down  a  map  of  the 
whole  system  on  the  same  scale.  If  tlie  orbit  of  Mercury  were 
represented  with  a  diameter  of  one-fourth  of  an  inch,  that  of 
Keptune  would  have  a  diameter  of  20  inches. 

With  the  exception  of  Neptune,  the  distances  of  the  eight 
majoi"  planets  proceed  in  a  tolerably  regular  progression,  the 
group  of  small  planets  taking  the  place  of  a  single  planet  in 
tlie  series.  Tlie  progression  is  known  as  the  law  of  Titius, 
from  its  iirst  proposer,  and  is  as  follows :  Take  the  series  of 
numbers  0,  3,  6,  12,  24,  48,  each  one  after  the  second  being 
formed  by  doubling  the  one  which  precedes  it.  Add  4  to 
each  of  these  numbers,  and  we  shall  have  a  series  of  numbers 
giving  very  nearly  the  relative  distances  of  the  planets  from 
the  sun.  The  following  table  shows  the  series  of  numbers  thus 
formed,  together  with  the  actual  distances  of  the  planets  ex- 
pressed on  the  same  scale,  the  distance  of  the  earth  being 
called  10 : 


riiinut. 


Mercury 

Venus 

Kaitli 

Mars 

Minor  jjlanets . 

Jupiter 

Saturn 

Uiaiius 

Neptuuo 


Nuiiibcrs  of  Titius. 

Actuftl  Distance. 

Error. 

0  +  4  =      4 

3.!> 

0.1 

a  +  4  =      7 

7.2 

0.2 

<;  +  4  =    10 

10.0 

0.0 

lli  +  4=    1(5 

15.2 

0.8 

24  +  4  =    28 

20  to  .T) 

4S  +  4  =   r.2 

r.2.0 

0.0 

!m;  +  4  =  100 

!».-i.4 

4.(; 

1!»2  +  4  ::^  llIC 

I'.M.i) 

4.1 

384  -1-  4  =  ;i88 

aoo.o 

87.4 

It  will  be  seen  that  before  the  discovery  of  Neptune  the 
agreement  was  so  close  as  to  suo-crestthe  existence  of  an  actual 
law  of  the  distances.  But  the  discovery  of  this  planet  in  1S46 
completely  disproved  the  supposed  law  ;  and  there  is  now  no 
reason  to  believe  that  the  })ro})ortions  of  the  solar  system  are 
the  result  of  any  exact  and  simple  law  whatever.  It  is  true 
that  many  ingenious  people  employ  themselves  from  time  to 
time  in  working  out  numerical  relations  between  the  distances 
of  the  planets,  their  masses,  their  times  of  rotation,  and  so  on, 


234  THE  SOL  Alt   SYSTEM. 

and  will  probably  continue  to  do  so;  because  the  number  of 
such  relations  which  can  be  made  to  come  somewhere  near  to 
exact  numbers  is  very  great.  This,  however,  does  not  indicate 
any  law  of  nature.  If  we  take  forty  or  fifty  numbers  of  any 
kind — say  the  3'ears  in  which  a  few  persons  were  born ;  their 
ages  in  years,  months,  and  days  at  some  particular  event  in 
their  lives ;  the  numbers  of  the  houses  in  which  they  live ;  and 
so  on — we  should  find  as  many  curious  relations  among  the 
numbers  as  have  ever  been  found  among  those  of  the  planet- 
ary system.  Indeed,  such  relations  among  the  years  of  the  lives 
of  gi'cat  actors  in  the  world's  history  w'ill  be  remembered  by 
many  readers  as  occurring  now  and  then  in  the  public  journals. 
Range  of  Planetary  Masses. — The  great  diversity  of  the  size 
and  mass  of  the  planets  is  shown  by  the  curious  fact,  that,  con- 
sidering the  sun  and  the  eight  planets,  the  mass  of  each  of  the 
nine  bodies  exceeds  the  combined  mass  of  all  those  which  are 
smaller  than  itself.  This  is  shown  in  the  following  simple  cal- 
culation. Suppose  tlie  sun  to  be  divided  into  a  thousand  mill- 
ions of  equal  parts,  one  of  which  })arts  we  take  as  the  unit  of 
weight:  then,  according  to  the  best  determinations  yet  made, 
the  mass  of  each  planet  will  be  that  used  in  the  following  cal- 
culation, in  which  each  mass  is  added  to  the  masses  of  all  the 
planets  Avhich  are  smaller  than  itself,  the  planets  being  taken 
in  tlic  order  of  their  masses,  beginning  with  the  smallest: 

Mass  of  Mercuiv 200 

Mass  of  Mars...*. 389 

Comhined  mass  of  Mercury  ami  Mars ri'VJ 

Mass  of  Venus 2,;}.")!$ 

Combined  mass  of  Mercmv.  Venus,  and  Mars 2,8!)2 

Mass  of  the  Earth ." 3,000 

Comhined  mass  of  the  four  inner  planets r»,!),')2 

Mass  of  Uranus 44, 2r)0 

Comhined  mass  of  five  i)Ianets .')(),202 

Mass  of  Nei)t ime Ti  1 , (>00 

Comhined  mass  of  six  planets 101,802 

Mass  of  Saturn 28.'>,r>80 

Combined  mass  of  seven  planets 387,382 

Mass  of  Jupiter i(r>4, 305 

Comhined  mass  of  all  the  i.i.mots 1,341,(587 

Mass  of  the  sun 1,000,000,000 


ASPECTS   OF  THE  PLANETS.  235 

It  will  be  seen  that  the  combined  mass  of  all  the  planets  is 
less  than  y^  that  of  the  sun  ;  that  Jupiter  is  between  two  and 
three  times  as  heavy  as  the  other  seven  planets  together;  Sat- 
urn more  than  twice  as  heavy  as  the  other  six ;  and  so  on. 

Aspects  of  the  Planets. — The  apparent  motions  of  the  plan- 
ets are  described  in  the  first  chapter  of  this  work ;  and  in  the 
second  chapter  it  is  shown  how  these  apparent  motions  result 
from  the  real  motions  as  laid  down  by  Copernicus.  The  best 
time  to  see  one  of  the  outer  planets  is  when  in  opposition  to 
the  sun.  It  then  rises  at  sunset,  and  passes  the  meridian  at 
midnight.  Between  sunset  and  midnight  it  will  be  seen  some- 
where between  east  and  south.  During  the  three  months  fol- 
lowing the  day  of  opposition,  the  planet  will  rise  from  three 
to  six  minutes  earlier  every  day.  A  month  after  opposition,  it 
will  be  two  to  three  hours  high  soon  after  sunset,  and  will  pass 
the  meridian  between  nine  and  ten  o'clock  at  night;  while 
three  months  after  opposition,  it  will  be  on  the  meridian  about 
six  in  the  evening.  Hence,  knowing  when  a  planet  is  in  op- 
position, a  spectator  will  know  pretty  nearly  where  to  look  for 
it.  Ilis  search  will  be  facilitated  by  the  use  of  a  star  map 
showing  the  position  of  the  ecliptic  among  the  stars,  because 
the  planets  are  always  very  near  the  ecliptic.  Indeed,  if  any 
bright  star  is  not  down  on  the  map,  he  may  feel  sure  that  it  is 
a  planet. 

In  describing  the  individual  planets,  we  give  the  times  when 
they  are  in  opposition,  so  that  the  reader  may  always  be  able 
to  recognize  them  at  favorable  seasons,  if  he  wishes  to  do  so. 

The  arrangement  of  the  planets,  with  their  satellites,  is  as 
follows : 


Inner  Qnonr.. 


OnTKn  GKonr  ok 
Gbkat  Planets. 


Mercury. 
Venus. 

Eartli,  with  its  moon. 
.  Mars. 

The  minor  plane    ,  or  asteroids. 

Jupiter,  with  4  moons. 
Saturn,  with  rings  and  8  moons. 
Uranus,  witli  4  moons. 
.  Neptune,  with  1  moon. 


236 


THE  SOLAR  SYSTEM. 


This  arraiigcinont  is  partly  exhibited  in  the  following  plan 
of  the  solar  system,  showing  the  relations  of  the  planetary  or- 
bits from  the  earth  outward.  The  scale  is  too  small  to  show 
the  orbits  of  Mercury  and  Venus. 


Q{^UilSnil}ll 


Fig.  64.— Orbits  of  the  planets  fmm  the  eaitli  otitwnrd,  showing  their  reiative  distances 
from  the  min  in  the  centre.  Tlie  positions  of  the  planets  are  near  those  which  they  oc- 
cupy in  1S77. 


THE  PROIOSrUERE.  237 


CHAPTER  11. 

tup:  sun. 

The  sun  presents  to  our  view  the  aspect  of  a  brilliant  globe 
32',  or  a  little  more  than  half  a  degree,  in  diameter.  To  give 
precision  to  our  language,  the  shining  surface  of  this  globe, 
which  we  see  with  the  eye  or  with  the  telescope,  and  which 
forms  the  visible  sun,  is  called  the  j^fiotosjyJiere.  Its  light  ex- 
ceeds in  intensity  any  that  can  be  produced  by  artificial 
means,  the  electric  light  between  charcoal  points  being  the 
only  one  which  does  not  look  absolutely  black  against  the  un- 
clouded sun.  Our  knowledge  of  the  nature  of  this  luminary 
commences  with  the  invention  of  the  telescope,  since  without 
this  instrument  it  was  impossible  to  form  any  conception  of 
its  constitution.  The  ancients  had  a  vac'ue  idea  that  it  was  a 
globe  of  lire,  and  in  this  they  were  more  nearly  right  than 
some  of  the  moderns ;  but  there  was  so  entire  an  absence  of 
all  real  foundation  for  their  opinions  that  the  latter  are  of  lit- 
tle interest  to  any  one  but  the  historian  of  philosophy.  We 
shall,  therefore,  commence  our  description  of  the  sun  with  a 
consideration  of  the  telescopic  researches  of  recent  times. 

§  1.  The  Photosphere. 

To  the  naked  eye  the  photos})here,  or  shining  surface  of  the 
sun,  presents  an  aspect  of  such  entire  uniformity  that  any  at- 
tempt to  gain  an  insight  into  its  structure  seems  hopeless. 
But  when  we  apply  a  telescope,  we  generally  lind  it  diversitied 
with  one  or  more  groups  of  dark-looking  spots ;  and  if  the  vis- 
ion is  good,  and  we  look  carefully,  we  shall  soon  see  that  the 
whole  bright  surface  presents  a  mottled  appearance,  looking 
like  a  fluid  in  which  ill-defined  rice-grains  are  suspended.  Per- 
haps the  most  familiar  idea  of  this  appearance  will  be  pre- 


238  THE  SOLAR  SYSTEM. 

sented  by  saying  that  the  siin  looks  like  a  plate  of  rice  soup, 
the  grains  of  rice,  however,  being  really  hnndreds  of  miles  in 
length.  Some  years  ago  Mr.  Nasmyth,  of  England,  examining 
the  sun  with  high  telescopic  powers,  announced  that  this  mot- 
tled appearance  seemed  to  him  to  be  produced  by  tlie  inter- 
lacing of  long,  narrow  objects  shaped  like  willow  leaves,  Avhich, 
running  and  crossing  in  all  directions,  form  a  net-work,  cover- 
ing the  entire  photosphere.  This  view,  though  it  has  become 
celebi-ated  through  the  very  great  care  which  Mr.  Nasmyth 
devoted  to  his  observations,  has  not  been  conlirmed  by  subse- 
quent observers. 

Among  tlie  most  crreful  and  laborious  telescopic  studies  of 
the  sun  recently  made  are  those  of  Professor  Langley.*  He 
has  a  line  telescope  at  his  command,  in  a  situation  where  the 
air  seems  to  be  less  disturbed  by  the  sun's  rays  than  is  usual 
in  other  localities.  According  to  his  observations,  when  the 
sun  is  carefully  examined,  the  mottling  which  we  have  de- 
scribed is  seen  to  be  caused  by  an  appearance  like  fleecy 
clouds  whose  outlines  are  nearly  inci^tinguishable.  We  may 
also  discern  numerous  faint  dots  on  the  white  background. 
Under  high  powers,  used  ^'n  favorable  moments,  the  surface 
of  any  one  of  the  fleecy  patches  is  resolved  into  a  congeries 
of  small,  intensely  bright  bodies,  irregularly  distributed,  which 
seem  to  be  suspended  in  a  comparatively  dark  medium,  and 
whose  definiteness  of  size  and  outline,  though  not  absolute,  is 
yet  striking,  by  contrast  with  the  vagueness  of  the  cloud-like 
forms  seen  before,  and  which  we  now  perceive  to  be  due  to 
their  aggregation.  The  "  dots  "  seen  before  are  considerable 
openings,  caused  l)y  the  absence  of  the  white  nodules  at  cer- 
tain points,  and  the  consequent  exposure  of  the  gray  medium 
which  forms  the  general  background.  These  openings  have 
been  called  pores.  Their  variety  of  size  makes  any  measure- 
ments nearly  valueless,  though  we  may  estimate  in  a  very 
rough  way  the  diameter  of  the  more  conspicuous  at  from  2" 
to  4". 

*  Professor  S.  P.  Langley,  Director  of  tlie  Obseivntory  at  Allegheny,  Pennsyl- 
vuniu. 


THE  PHOTOSPHERE.  239 

In  moments  when  the  definition  is  very  fine,  the  bright  nod- 
ules or  rice-grains  are  found  to  be  made  up  of  chisters  of  mi- 
nute points  of  light  or  "granules,"  about  one-third  of  a  second 
in  diameter.  These  have  also  been  seen  around  the  edges  of 
the  pores  by  Secchi,  who  estimated  their  magnitude  as  even  less 
than  that  assigned  by  Langley.  The  fact  that  these  points  are 
aggregated  into  little  clusters,  which  ordinarily  present  the  ap- 
pearance of  rice-grains,  gives  the  latter  a  certain  irregularity  of 
outline  which  has  been  remarked  by  Mr,  Iluggins.  Thus,  there 
appear  to  be  three  orders  of  aggregation  in  tlic  brighter  re- 
gions of  the  photosphere :  cloud-like  forms  which  can  be  easi- 
ly seen  at  any  time ;  rice-grains  or  nodules,  into  which  these 
forms  are  resolved,  and  which  can  always  be  seen  with  a  fair 
telescope  under  good  definition  ;  and  granules  which  make  up 
the  rice -grains.  This  structure  of  the  rice -grains  has  been 
seen  only  by  Professor  Langley. 

If  we  carefully  examine  the  sun  with  a  very  dark  smoked 
glass,  we  sliall  find  that  the  disk  is  brightest  at  the  centre, 
shading  off  on  all  sides  towards  the  limb.  Careful  compari- 
sons of  the  intensity  of  radiation  of  different  parts  of  the  disk 
show  that  this  diminution  near  the  limb  is  common  to  all  the 
rays,  whether  those  of  heat,  of  light,  or  of  chemical  action. 
The  most  recent  measures  of  the  heat  rays  were  made  by 
Langley  by  means  of  a  thermo-electric  pile,  those  of  the  light 
rays  by  Pickei'ing,*  and  those  of  the  chemical  rays  by  Vogel.f 
The  intensities  of  these  several  radiations  at  different  distances 
from  the  centre  of  the  disk  as  thus  determined  are  shown  in 
the  table  on  the  following  page.  Tlic  intensity  at  the  centre 
is  always  supposed  100.  The  first  column  gives  the  distance 
from  the  centre  in  fractions  of  the  sun's  radius,  whicli  is  sup- 
posed unity.  Thus,  the  first  line  of  the  table  corresponds  to 
the  centre;  the  last  to  tlie  edge.  Professor  Langley's  meas- 
ures do  not,  however,  extend  to  the  extreme  edge. 


*  Professor  E.  C.  Pickering,  director  of  the  Harvard  Observatory,  Cambridge, 
Massachusetts. 

t  Dr.  Ileimann  C.  Vogel,  formerly  astronomer  at  Botiikamp,  now  of  the  Solar 
Obsen'atory  in  Potsdam,  Prussia. 


240 


THE  SOLAR  SYSTEM. 


Oiatanre  from 

Hent  Rnj-s 

LlRht 

Cliemical  Uays 

Centre  of  the  Sun. 

(Lniigli'.v). 

(Pit^kerinj?). 

(Vogel). 

.00 

100 

100 

100 

.125 

•  •  •  • 

99 

100 

,2r> 

99 

97 

98 

.;^75 

.*■  • 

94 

95 

.50 

95 

91 

90 

A)2-) 

■  •  •  * 

HC 

81 

.7-, 

86 

79 

(!() 

.85 

•  ■  •  ■ 

C9 

48 

.!»5 

•  •  •  • 

55 

25 

.!)(5 

f.2 

*  •  •  • 

2;{ 

.98 

50 

*  ■  ■  ■ 

18 

1.00 

.... 

37 

la 

It  will  be  seen  that  near  tlie  edge  of  the  disk  the  chemical 
rays  fall  off  most  rapidly,  the  light  rays  next,  and  the  heat 
rays  least  of  all.  Roughly  speaking,  each  square  minute  near 
the  limb  of  the  sun  gives  about  half  as  much  heat  as  at  the 
centre,  about  one-third  as  much  light,  and  less  than  one-seventh 
as  many  photographic  rays.  Of  the  cause  of  this  degradation 
of  light  and  heat  towards  the  limb  of  the  sun  no  doubt  has 
been  entertained  since  it  was  first  investigated.  It  is  found  in 
the  absorption  of  the  rays  by  a  solar  atmosphere.  The  sun 
being  a  globe  surrounded  by  an  atmosphere,  the  rays  which 
emanate  from  the  photosphere  in  a  horizontal  dii'cction  have 
a  greater  thickness  of  atmosphere  to  pass  through  than  those 
which  strike  out  vertically ;  while  the  former  are  those  we 
see  near  the  edge  of  the  disk,  and  the  latter  near  the  centro. 
The  different  absorptions  of  different  classes  of  rays  corre- 
spond exactly  to  tliis  supposition,  it  being  known  that  the 
more  refrangible  or  chemical  rays  are  most  absorbed  by  va- 
pors, and  the  heat  rays  the  least. 

From  this  it  follows  that  we  get  but  a  fraction — perhaps  a 
small  fraction — of  the  liffht  and  heat  actuallv  emitted  by  the 
sun ;  and  that  if  the  latter  had  no  atmosphere,  it  would  be 
much  hotter,  much  brighter,  and  bluer  in  color,  than  it  actually 
is.  The  total  amount  of  absorption  has  been  very  differently 
estimated  by  different  authorities,  Laplace  supposing  it  might 
be  as  much  as  eleven  -  twelfths  of  the  whole  amount.  The 
smaller  estimates  are,  however,  more  likely  to  be   near  the 


THE  PHOTOSPHERE.  241 

truth,  there  being  no  good  reason  for  holding  that  more  than 
half  the  rays  are  absorbed.  Tliat  is,  if  the  sun  had  no  atmos- 
phere, it  might  be  tMaco  as  bright  and  as  hot  as  it  actually  is, 
but  would  not  be  likely  to  be  three  or  four  times  so.  Profess- 
or Langley  suggests  that  the  glacial  epoch  may  have  been  due 
to  a  greater  absorption  of  the  sun's  heat  by  its  atmosphere  in 
some  past  geological  age. 

A  ver}'  important  physical  and  astronomical  problem  is  that 
of  measuring  the  total  amount  of  heat  radiated  by  the  sun  to 
the  earth  during  any  period  of  time  —  say  a  day  or  a  year. 
The  question  admits  of  a  perfectly  definite  answer,  but  there 
are  two  difficulties  in  the  way  of  obtaining  it;  one,  to  distin- 
guish between  the  heat  coming  from  the  sun  itself,  and  that 
coming  from  the  atmosphere  and  surrounding  objects;  the 
other,  to  allow  for  the  absorption  of  the  solar  lieat  by  our  at- 
mosphere, which  nnist  be  done  in  order  to  determine  the  to- 
tal quantity  emanating  from  the  sun.  The  most  successful 
experiments  for  this  purpose  are  those  of  Pouillet  and  of 
Sir  John  Ilerschel.  The  results  obtained  by  the  former  may 
be  expressed  thus :  if  the  air  were  out  of  the  way,  and  a  sheet 
of  ice  were  so  held  tliat  the  sun's  rays  should  fall  u})on  it  per- 
pendicularly, and  be  all  absorbed,  the  ice  would  melt  away  at 
the  rate  of  14^  inches  in  24  hours.  Since  the  sun  is  part  of 
the  time  below  the  horizon,  and  \r-,  not  perpendicular  to  more 
than  a  single  point  of  the  earth's  surface  when  above  it,  the 
average  amount  of  ice  which  would  be  melted  over  the  whole 
earth  is  only  a  fraction  of  this,  namely,  3.62  inches  per  day, 
or  something  more  than  100  feet  per  year. 

Attempts  have  been  made  to  determine  the  temperature  of 
the  sun  from  the  amount  of  heat  whicli  it  radiates,  but  the 
estimates  have  varied  very  widely,  owing  to  the  uncertainty 
respecting  the  law  of  radiation  at  high  temperatures.  By  sup- 
posing the  radiation  proportional  to  the  temperature,  Secchi* 
finds  the  latter  to  be  several  million  degrees,  while,  by  taking 
another  law  indicated  by  the   experiments  of   Dulong   and 

*  Father  Aiigelo  Secclii,  Director  of  the  Obseivator}'  at  liome. 

17 


242  THE  SOLAR  SYSTEM. 

Petit,  others  find  a  temperature  not  many  times  exceeding 
that  of  a  reverl)eratory  furnace.  For  the  temperature  of  the 
photosphei'e,  it  seems  Hkely  tliat  tlie  lower  estimates  are  more 
nearly  right,  heing  founded  on  an  experimental  law ;  but  the 
temperature  of  the  interior  must  be  immensely  higher. 

§  2.  The  Solar  Spots  and  Rotation. 

Even  the  poor  telescopes  made  by  the  contemporaries  of 
Galileo  could  hardly  be  directed  to  the  sun  many  times  with- 
out one  or  more  spots  being  seen  on  his  surface.  Whatever 
credit  may  be  due  for  a  discovery  which  required  neither  in- 
dustry nor  skill  should,  by  the  rule  of  modern  science  already 
referred  to,  be  awarded  to  Fabritius  for  the  discovery  of  the 
solar  spots.  This  observer,  otherwise  unknown  in  astronomy, 
made  known  the  existence  of  the  solar  spots  early  in  IGll — 
a  year  after  Galileo  began  to  scan  the  heavens  with  his  tel- 
escope. His  discovery  was  followed  up  by  Galileo  and  Schei- 
ner,  by  whom  the  first  knowledge  of  the  nature  of  the  spots 
was  acquired. 

The  first  idea  of  Scheiner  was  that  the  spots  were  small 
planets  in  the  neighborhood  of  the  sun  ;  but  this  was  speedily 
dis})roved  by  Galileo,  who  showed  that  they  must  be  on  the 
surface  of  the  sun  itself.  The  idea  of  the  sun  beinc:  affected 
with  any  imperfection  so  gross  as  a  dark  spot  was  repugnant 
to  the  ecclesiastical  philosophy  of  the  times,  and  it  is  not  un- 
likely that  Sclieiner's  explanation  was  suggested  by  the  desire 
to  save  the  perfection  of  our  central  luminary. 

A  vei-y  little  observation  showed  that  the  spots  had  a  regu- 
lar motion  across  the  disk  of  the  sun  from  east  to  west,  occu- 
pying about  12  days  in  the  transit.  A  spot  generally  appeared 
first  on  or  near  the  east  limb,  and,  after  12  or  14  days,  disap- 
peared at  the  west  limb.  At  the  end  of  another  14  days  or 
more  it  reappeared  at  the  east  limb,  uidess  in  the  mean  time 
it  had  vaTiished  from  sight  entirely.  The  spots  were  found 
not  to  be  permanent  objects,  but  to  come  into  existence  from 
time  to  time,  and,  after  lasting  a  few  days,  weeks,  or  months, 
to  disappear.      But  so  long  as  they  lasted,  they  always  ex- 


THE  SOLAR  SPOTS  AND   liOTATJOX. 


243 


liibited  the  motion  just  described,  and  it  was  tlience  inferred 
that  the  sun  rotated  on  his  axis  in  about  25  days. 

The  astronomers  of  tlie  seventeenth  and  ei<;jliteenth  centuries 
used  a  method  of  observing  tlie  sun  whicli  will  often  be  found 
convenient  for  seeing  the  spots  when  one  has  not  a  telescope 
su[)plied  with  dark  glasses  at  his  disposal.  Take  an  ordinary 
good  spy -glass,  or,  indeed,  a  telescope  of  a!iy  size,  and  point 


Fio.  C5.— Man  holding  telescope,  to  show  sun  on  screen. 

it  at  the  sun.  To  save  the  eyes,  the  right  direction  may  be 
found  by  holding  a  piece  of  paper  closely  in  front  of  the  eye- 
piece :  when  the  sun  shines  through  the  telescope  on  this  pa- 
per, the  pointing  is  nearly  right.  The  telescope  should  be  at- 
tached to  some  movable  support,  so  that  its  })ointing  can  be 
changed  to  the  different  directions  of  the  sun,  and  should  pass 
through  a  perforation  in  some  sort  of  a  screen,  so  that  the 
sun  cannot  shine  in  front  of  the  telescope  except  by  passing 


244 


THE  SOLAR  SYSTEM. 


tlirongh  it.  An  opening  in  a  windovv-sliiitter  will  answer  a 
good  purpose,  only  the  rays  ninst  not  liave  to  pass  throngh  the 
glass  of  the  window  in  order  to  reach  the  telescope.  Draw 
out  the  eye-piece  of  the  instrument  about  the  eighth  of  an 
inch  beyond  the  proper  point  for  seeing  a  distant  object. 
Then,  holding  a  piece  of  white  paper  before  the  eye-i)iece  at 
a  distance  of  from  G  to  12  inches,  an  image  of  the  sun  will  be 
thrown  upon  it.  The  distance  of  the  paper  must  be  adjusted 
to  the  distance  the  eye-piece  is  drawn  out.  The  farther  wo 
draw  out  the  eye  -  piece,  the  nearer  the  best  image  will  be 
formed.  Having  adjusted  everything  so  that  the  edge  of  the 
sun's  image  shall  be  sharply  defined,  one  or  more  spots  can 
generally  be  seen.  This  method,  or  something  similar  to  it,  is 
often  used  in  observing  eclipses  and  transits  of  Mercury,  and 
is  very  convenient  when  it  is  desired  to  show  an  enlarged  im- 
age of  the  sun  to  a  number  of  spectators. 

When  powerful  telescopes  were  applied  to  the  sun,  it  Avas 
found  that  the  spots  were  not  merely  the  dark  patches  which 
they  lirst  appeared  to  be,  but  that  they  comjirised  two  well- 


Fio.  60 Sulur  spot,  afler  Seccbi. 

marked  portions.  The  central  part,  called  the  nmhra  or  nu- 
cleus^ is  the  darkest,  and  is  surrounded  by  a  border,  interme- 
diate in  tint  between  the  darkness  of  the  spot  and  the  brill- 


TUE  SOLAR  SPOTS  AN  J)  ROT  ATI  OX.  245 

iancy  of  tlie  solar  surf.acc.  This  border  is  termed  tlie  penum- 
bra. Ordinarily  it  ajipears  of  a  uniforin  gray  tint.  ]]ut  wlien 
carefully  examined  with  a  good  telescope  in  a  very  steady  at- 
mosphere, it  is  found  to  be  striated,  looking,  in  fact,  much  like 
the  bottom  of  a  thatched  roof,  the  separate  straws  being  di- 
rected towards  the  interior  of  the  spot.  This  appearance  is 
shown  in  the  figure. 

The  spots  are  extremely  irregtilar  in  form  and  unequal  in 
size.  They  are  very  generally  seen  in  groups  —  sometimes 
two  or  more  combined  into  a  single  one ;  and  it  frequently 
happens  that  a  large  one  breaks  up  into  several  smaller  ones. 
Their  duration  is  also  extremely  variable,  ranging  from  a  few 
days  to  periods  of  several  months. 

Until  about  a  century  ago,  it  was  a  question  whether  the 
spots  were  not  dark  patches,  like  scoria,  Hoating  on  the  molten 
surface  of  the  photosphere.  Wilson,  a  Scotch  observer,  how- 
ever, found  that  they  ai)peared  like  cavities  in  the  photosphere, 
the  dark  part  being  really  lower  than  Mie  bright  surface  around 
it.  As  a  spot  approached  the  edge  of  the  disk,  he  found  that 
the  penumbra  grew  disproportionately  narrow  on  the  side 
nearest  to  the  sun's  centre,  showing  that  this  side  of  it  was 
seen  at  a  smaller  angle  than  the  otlier.  This  effect  of  per- 
spective is  shown  in  Fig.  67,  where,  near  the  sun's  limb,  the 
side  of  the  penumbra  nearest  us  is  hidden  by  the  photosphere. 
That  the  spots  are  cavities  is  also  shown  by  the  fact  that 
when  a  lai-ge  spot  is  exactly  on  the  edge  of  the  disk  a  notch 
is  sometimes  seen  there.  The  shaded  ])e]iumbra  seems  to 
form  the  sides  of  the  cavitv,  while  the  umbra  is  the  invisible 
bottom. 

Tiiese  observations  gave  rise  to  the  celebrated  theory  of 
Wilson,  which  is  generally  connected  with  the  name  of  ller- 
schel,  who  developed  it  more  fully.  The  interior  of  the  suti 
is,  by  this  theory,  a  cool,  dark  body,  sui'rounded  by  two  layers 
of  clouds.  The  outer  layer  is  intensely  brilliant,  and  forms 
the  visil)le  photosphere,  while  the  inner  layer  is  darker,  and 
foi'ms  the  uml)ra  around  the  spots.  The  latter  are  simply 
openings  through  these    clouds,  which    form    from    time    to 


246 


THE  SOLAR  SYSTEM. 


PiQ.  67.— Chauges  iu  the  aspect  of  a  solar  spot  as  it  crosees  the  buu'b  disk,  showiug  it  to  be 

a  cavity  in  the  photosphere. 

time,  and  tlironc;li  wliicli  we  see  tlie  dark  body  in  the  interior. 
Anxious  tliat  this  body  should  serve  sonic  esj)ecial  purpose  iu 
tlie  economy  of  creation,  tliey  peopled  it  with  intellijjjent  be- 
ings, who  were  protected  from  the  fierce  radiation  of  the  pho- 
tosphere by  the  layer  of  cool  clouds,  but  were  denied  every 
view  of  the  universe  -ith-out,  except  such  glimpses  as  they 
might  obtain  through  the  occasional  openings  in  the  photo- 
sphere, which  we  see  as  spots. 

Leaving  out  the  fancy  of  living  beings,  this  theory  a(!count- 
ed  very  well  for  appeai'ances.  That  the  photosphere  (H)uld  not 
be  absolutely  and  wholly  solid,  liquid,  or  gaseous  seemed  evi- 
dent from  the  nature  of  the  spots.  If  it  were  solid,  the  latter 
could  not  be  in  such  a  constant  state  of  chance  n**  we  see 


THE  SOLAR  SPOTS  AND  ROTATION.  247 

them ;  wliile  if  it  were  liquid  or  gaseous,  these  cavities  could 
not  continue  for  niontlis,  as  they  were  sometimes  seen  to,  be- 
cause the  liquid  or  gaseous  matter  would  rush  in  from  all 
sides,  and  fill  them  up.  Tiie  only  hypothesis  that  seemed  left 
open  to  Ilerschel  was  that  the  photosphere  consisted  of  clouds 
floating  in  an  atmosphere.  As  the  sides  of  the  cavities  looked 
comparatively  darK,  the  conclusion  seemed  inevitable  that  the 
brilliancy  of  the  photosphere  was  only  on  and  near  the  sur- 
face ;  and  as  the  bottom  of  the  cavity  looked  entirely  dark, 
the  conclusion  that  the  sun  had  a  dark  interior  seemed  una- 
voidable. 

The  discovery  of  the  conservation  of  force,  and  of  the  mut- 
ual convertibility  of  heat  and  force,  was  fatal  to  this  theory. 
Such  a  sun  as  that  of  Ilerschel  would  have  cooled  off  entirely  in 
a  few  days,  and  then  we  should  receive  neither  light  nor  heat 
from  it.  A  continuous  flood  of  heat  such  as  the  sun  has  been 
radiating  for  thousands  of  years  can  be  kept  up  only  by  a  con- 
stant expenditure  of  force  in  some  of  its  forms ;  but,  on  Iler- 
schel's  theory,  the  supply  necessary  to  meet  this  expenditure 
was  impossible.  E.en  if  the  heat  of  the  photosphere  could 
be  ke])t  up  by  any  agency,  it  would  be  constantly  conveyed  to 
the  interior  by  conduction  and  radiation  ;  so  that  in  time  the 
whole  sun  would  become  as  hot  as  the  photosphere,  and  its 
inhabitants  would  be  destroyed.  In  the  time  of  Ilerschel  it 
was  not  deemed  necessary  that  the  sun  should  be  a  very  hot 
body,  the  heat  received  from  his  rays  lieing  supposed  by  many 
to  be  generated  b}'  their  passage  through  our  atmosphere. 
The  photosphere  was,  therefore,  supposed  to  bo  simply  })ho8- 
pliorescent,  not  hot.  This  idea  is  still  entertained  by  many 
educated  men  who  have  not  made  themselves  ac(piaintod  with 
the  laws  of  heat  discovered  during  the  present  century.  We 
may,  therefore,  remark  that  it  is  completely  untenable.  One 
of  the  best  established  results  of  these  laws  is  that  the  surface 
of  the  sun  is  intensely  hot,  prol)ably  much  hotter  tiian  any  re- 
verbcratory  furnace.  The  great  question  in  the  present  state 
of  science  is,  how  the  supply  of  heat  is  nuiintained  against 
such  immense  loss  bv  radiation. 


248  THE  SOLAR  SYSTEM. 

§  3.  Periodicity  of  the  Spots. 

The  careful  observations  of  tlie  solar  spots  which  have  been 
made  during  the  last  century  seem  to  indicate  a  period  of 
about  eleven  years  in  the  spot-producing  activity  of  the  sun. 
During  two  or  three  years  the  spots  are  larger  and  more  nu- 
merous than  on  the  average ;  they  then  begin  to  diminish, 
and  reacli  a  minimum  live  or  six  years  after  the  maximum. 
Anotiier  six  years  brings  the  return  of  the  maximum.  The 
intervals  are,  however,  somewhat  irregular,  and  further  obser- 
vations are  required  before  the  law  of  this  period  can  be  lixed 
with  certainty.  An  idea  of  the  evidence  in  favor  of  the  pe- 
riod may  be  formed  from  some  I'csults  of  the  observations  of 
Scliwabe,  a  German  astronomer,  who  systematically  observed 
the  sun  during  a  large  part  of  a  long  life.  One  of  his  meas- 
ures of  the  spot-producing  power  was  the  number  of  days  on 
which  he  sa  v  the  sun  without  spots  i'l  the  course  of  each 
year.     The  following  are  some  of  his  results: 

From  1828  to  1831,  sup  without  spots  on  only  1  day. 

In  18:?M,  "  "  "  i;5!»days. 

From  18;5(;  to  1840,  "  "  "  ;?  days. 

In  I84;5,  "  "  "  147  days. 

From  1847  to  18:)1,  "  "  "  'J  days. 

In  18r.(;,  "  "  "  1!»;}days. 

From  18r)8  to  18(;i,  "  "  "  no  day. 

In  18(;7,  "  "  '•  r.ir)  days. 

We  see  that  the  sun  was  remarkably  free  from  spots  in  the 
years  1833,  1843,  1850,  and  18()7,  about  half  the  time  no  con- 
siderable spot  being  visible.  This  recurrence  of  the  period 
lias  been  traced  back  by  Dr.  Wolf,  of  Zurich,  to  the  time  of 
(Jalileo,  and  its  average  lengtli  is  about  11  years  1  month. 
The  years  of  fewe^^t  sun  spots  during  the  present  century  were 
1810,  1823,  1833,  1844,  185G,  and  ISOT.  Continuing  the 
series,  we  may  expect  very  few  s})ot8  in  1878,  1881),  etc.  The 
years  of  greatest  ])roduction  of  sjiots  were  1804,  1810,  1829, 
1837,  1848,  1800,  and  1870,  from  wliich  we  nuiy  conclude 
that  1882, 1893,  etc.,  will  be  years  of  numerous  sun-spots. 


rERIODICITY  OF  THE  SPOTS.  249 

The  observations  of  Schwabe  and  the  researches  of  Wolf 
seem  to  have  placed  the  existence  of  this  period  beyond  a 
doubt;  but  no  satisfactory  explanation  of  its  cause  has  yet 
been  given.  When  first  noti'^ted,  its  near  approach  to  the  pe- 
riod of  revolution  of  Jupiter  naturally  led  to  the  belief  that 
there  was  a  connection  between  the  two,  and  that  the  attrac- 
tion of  the  largest  planet  of  the  system  produced  some  disturb- 
ance in  the  sun,  which  was  greater  in  perihelion  than  in  aphe- 
lion. But  this  connection  seems  to  be  disproved  hy  the  fact 
that  the  sun-s})ot  period  is  at  least  six  months,  and  perhaps  a 
year,  shorter  than  the  revolution  of  Jupiter.  It  is  therefore 
probable  that  the  periodicity  in  cpiestion  is  not  due  to  any  ac- 
tion outside  the  sun,  but  is  a  result  of  some  law  of  solar  action 
of  which  we  are  as  yet  ignorant. 

There  are  certain  su[)posed  connections  of  the  sun-spot  pe- 
riod with  terrestrial  ])henomena  which  are  of  interest.  Sir 
William  Ilerschel  collected  quite  a  mass  of  statistics  tending  to 
show  that  there  was  an  intimate  connection  between  the  num- 
ber of  sun-spots  and  the  price  of  corn,  the  latter  being  low 
when  there  were  few  spots,  and  high  when  they  were  more 
numerous.  Ilis  conctusion  was  that  the  fewer  the  spots,  the 
more  favorable  the  solar  rays  to  the  growth  of  the  crops. 
This  theory  has  not  been  confirmed  hy  subsequent  observa- 
tion. There  is,  however,  some  reason  to  believe,  from  the 
researches  of  Professors  Levering  and  Loonn's,  that  the  fre- 
quency of  auroras  and  of  magnetic  disturbances  is  subject  to 
a  period  corresponding  to  that  of  sun-spots,  these  occurrences 
being  most  frequent  when  the  spots  are  most  numerous.  Pro- 
fessor Looniis  considers  the  coincidence  to  be  [.retty  well 
|>r(»vod,  while  Professor  Lovering  is  iiiore  cautious,  and  waifs 
for  further  research  before  coming  to  a  positive  conclusion. 
Tiie  occurrence  of  great  auroras  in  1859  and  ISTO-'Tl  was 
strikingly  accordant  with  the  theory. 

§  4.   Lair  of  Rotation  of  th>'  San. 

P)etween  the  vears  1S43  and  18()l,a  verv  careful  series  of 
ob8eivati>ns  of  the  positions  and  motions  of  the  solar  spots 


250  THE  SOLAR   SYSTEM. 

was  made  by  Mr.  Carrington,  of  England,  with  a  view  of  de- 
ducing tlie  exact  time  in  wliich  the  sun  rotates  on  his  axis. 
These  observations  led  to  tlie  remarkable  result  that  the  time 
of  rotation  shown  by  the  spots  was  not  the  same  on  all  parts 
of  the  sun,  but  that  the  equatorial  regions  seemed  to  perform 
a  revolution  in  less  time  than  those  nearer  the  poles.  Near 
the  equator  the  period  was  about  25.3  days,  while  it  was  a 
day  longer  in  30°  latitude.  Moreover,  the  period  of  rotation 
seems  to  be  different  at  different  times,  and  to  vary  with  the 
frequency  of  the  spots.  But  the  laws  of  these  variations  are 
not  yet  established.  In  consequence  of  their  existence,  we 
cannot  fix  any  definite  time  of  rotation  for  the  sun,  as  we  can 
for  the  earth  and  for  some  of  the  planetG.  It  varies  at  dif- 
ferent times,  and  under  different  circumstances,  from  25  to 
2G^  days. 

The  cause  of  these  variations  is  a  subject  on  which  there  is 
yet  no  general  agreement  among  those  who  have  most  care- 
fully investigated  the  subject.  Zollner*  and  Wolf  see  in  the 
general  motions  of  the  spots  traces  of  currents  moving  from 
both  poles  of  the  sun  towards  the  equator.  The  latter  con- 
siders that  t'""  eleven -year  spot -period  is  associated  with  a 
flood  of  liq  id  or  gaseous  matter  thrown  up  at  the  poles  of 
the  sun  about  once  in  eleven  years,  and  gradually  finding  its 
way  to  the  equator.  Z(")llner  adopts  the  same  theory,  and  has 
submitted  it  to  a  mathematical  analysis,  the  basis  of  which  is 
that  the  sun  has  a  solid  crust,  over  which  runs  the  fluid  in 
which  the  spots  are  formed.  The  current  springs  up  near 
the  poles,  and,  starting  towards  the  equator  without  aiiy  rota- 
tion, is  acted  on  by  the  friction  of  the  revolving  crust.  I>y 
this  friction  the  crust  continua'ly  tends  to  carry  the  fluid  with 
it.  The  nearer  the  current  approaches  the  equator,  the  more 
rapid  the  rotation  of  the  crust,  owing  to  its  greater  distance 
from  the  axis.  The  frictioji  acts  so  slowly  that  tlie  current 
reaches  the  equator  before  it  takes  up  the  motion  of  the  crust. 
On  this  hypothesis,  the  crust  of  the  sun  really  revolves  in 

*  Dr.  J.  C  F.  Zulliier,  Professor  in  the  University  of  Leiijsic. 


THE  SUN'S  SUBROUNDINGS.  251 

about  25  days ;  and  the  reason  that  the  fluid  which  covers  it 
revolves  more  slowly  at  a  distance  from  the  sun's  equator  is 
that  it  has  not  yet  taken  up  this  normal  velocity  of  rotation. 

Tliis  explanation  of  the  seeming  paradox  that  the  equatorial 
regions  of  the  sun  perform  their  revolution  in  a  shorter  time 
than  those  parts  nearer  the  poles,  cannot  be  regarded  as  an  es- 
tablished scientific  theory.  It  is  mentioned  as  being,  so  far  as 
the  writer  is  aware,  the  most  completely  elaborated  explana- 
tion yet  offered.  It  is  possil)le  that  the  spots  have  a  proper 
motion  of  their  own  on  the  solar  surface,  and  that  this  is  the 
reason  of  the  apparent  difference  in  the  time  of  rotation  in 
different  latitudes.  Yet  atiother  theory  of  the  subject  is  that 
of  Faye,*  who  maintains  that  these  differences  in  the  rates  of 
rotation  are  due  lo  ascending  and  descending  currents,  as  will 
be  more  fully  explained  in  presenting  his  views.  But  we  here 
touch  upon  questions  which  science  is  as  yet  far  from  being 
in  a  condition  to  answer. 

§  5.  The  Suns  Surroundings. 

If  the  sun  had  never  been  examined  with  any  other  instru- 
ment than  the  telescope,  nor  been  totally  eclipsed  by  the  inter- 
vention of  the  moon,  we  should  not  have  formed  any  idea  of 
the  nature  of  the  operations  going  on  at  his  surface ;  but  we 
might  have  been  better  satisfied  that  we  had  a  complete  knowl- 
edge of  his  constitution.  Indeed,  it  is  remarkable  thiit  mod- 
ern science  has  shown  us  more  mvstcries  in  the  sun  than  it  has 
explained  ;  so  that  we  And  ourselves  farther  than  before  from 
a  satisfactory  explanation  of  solar  phenomena.  When  the  an- 
cients suj)posed  the  sun  to  be  a  globe  of  molten  iron,  they  had 
an  explanation  which  quite  satisfied  the  requirements  of  the 
science  of  their  times.  Tin  spots  were  no  mystery  to  (Talileo 
and  Scheiner,  being  simply  dark  places  in  the  ])hotos]>]iere. 
Ilerschel's  explanation  of  them  was  quite  in  accord  with  the 
science  of  his  time,  and  he  may  be  regarded  as  the  latest  man 
who  has  held  a  theory  of  the  physical  constitution  of  the  sun 

*  Mr.  II.  E.  Fiive,  member  of  the  Fiend)  Acadeinj  of  Sciences. 


252  THE  SOLAR  SYSTEM. 

wliich  was  really  satisfactory  at  the  time  it  was  propounded. 
AVe  have  shown  how  his  theory  was  refuted  hy  the  discovery 
of  tlie  conservation  of  force ;  we  ha,ve  now  to  see  what  per- 
plexing phenomena  have  been  revealed  in  recent  times. 

Phenomena  during  Total  Eclipses.  —  If,  during  tlie  progress 
of  a  total  eclipse,  the  gradually  diminishing  crescent  of  the 
sun  is  watched,  nothing  remarkable  is  seen  until  very  near  the 
moment  of  its  total  disappearance.  But,  as  the  last  ray  of  sun- 
light vanishes,  a  scene  of  unexampled  beauty,  grandeur,  and  im- 
pressivcness  breaks  upon  the  view.  The  globe  of  the  moon, 
black  as  ink,  is  seen  as  if  it  were  hanging  in  mid-air,  surround- 
ed 1)}'  a  crown  of  soft,  silvery  light,  like  that  which  tlie  old 
painters  used  to  depict  around  the  heads  of  saints.  Besides 
tiiis  '•  corona,"  tongues  of  rose-colored  tlame  of  the  most  fan- 
tastic forms  shoot  out  from  various  points  around  the  edge  of 
the  lunar  disk.  Of  these  two  a])[)earances,  the  corona  was  no- 
ticed at  least  as  far  back  as  the  time  of  Kepler ;  indeed,  it  was 
not  possible  for  a  total  eclipse  to  ha])pen  without  the  specta- 
tors seeing  it.  But  it  is  only  within  a  century  that  the  at- 
tention of  astrono!ners  has  been  directed  to  the  rose-colored 
Hames,  although  an  observation  of  them  was  recorded  in  the 
Bhilosopliical  Transactions  nearly  two  centuries  ago.  They 
are  known  by  the  several  names  of  "  flames,"  ""  prominences," 
aiul  ''  protuberances." 

The  descriptions  wliich  have  been  given  of  the  corona,  al- 
though differing  in  many  details,  have  a  general  resemblance. 
Ilalley'S  description  of  it,  as  seen  during  the  total  eclipse  of 
1715,  is  as  follows: 

"A  few  seconds  before  the  sun  was  all  hid,  thei-e  discovered 
itself  round  the  moon  a  luminous  ring  about  a  digit,  or  per- 
ha])s  a  tenth  ])art  of  the  moon's  diameter,  in  breadth.  It  M'as 
of  a  pale  whiteness,  or  ratlier  pearl-coh)r,  seeming  to  me  a  lit- 
tle tinged  Mith  the  colors  of  the  ii'is,  and  to  be  concentric 
with  tlie  moon." 

The  more  careful  and  elabo.aio  observations  of  recent  times 
show  tliat  tlie  corona  has  not  the  circular  form  which  was  for 
nierly  ascribed  to  it,  but  that  it  is  quite  irregular  in  its  out- 


THE  SUN'S  SUliEOUXDINGS.  253 

line.  Sometimes  its  form  is  more  nearly  square  than  round, 
the  corners  of  the  square  being  about  45°  of  solar  latitude, 
and  the  sides,  therefore,  corresponding  to  the  poles  and  the 
equator  of  the  sun.  This  square  appearance  does  not,  how- 
evei',  arise  from  any  regularity  of  form,  but  from  tlie  fact  that 
the  corona  seems  brighter  and  higher  half  v/ay  between  the 
poles  and  the  equator  of  the  sun  than  it  does  near  those  points. 


Fio.  08. — Total  eclipse  of  the  snn  as  seon  at  Des  Moines,  Iowa,  August  7tli,  1809.  Drawn 
by  Professor  J,  K.  Eastman.  The  letters,  a,  b,  c,  etc.,  mark  the  positioua  of  the  prom- 
inences. 

These  prominent  portions  sometimes  seem  like  rays  shooting 
out  from  the  sun.  The  corona  is  always  brightest  at  its  base, 
gradually  shading  ofP  toward  the  outer  edge.  It  is  impossi- 
ble to  say  with  certainty  how  far  it  extends,  but  there  is  no 
doubt  that  it  has  ueen  seen  as  far  as  one  semidiameter  from 
the  moon's  limb. 


254  TEE  SOLAR  SYSTEM. 

The  corona  was  formerly  supposed  to  be  an  atmosphere 
eitlier  of  the  moon  or  of  the  sun.  Thirty  or  forty  years  ago, 
the  most  plausible  theory  was  that  it  was  a  solar  atmos})here, 
and  that  the  red  protuberances  were  clouds  floating  in  it. 
That  the  corona  could  be  a  lunar  atmosphere  was  completely 
disproved  by  its  irregular  outline,  for  the  atmosphere  of  a 
body  like  the  moon  would  necessarily  spread  itself  around  in 
nearly  uniform  layers,  aiul  could  not  be  piled  up  in  some 
quarters,  as  the  matter  of  the  corona  is  seen  to  be.  We  shall 
soon  see  that  there  is  no  doubt  about  the  corona  beijig  some- 
thing suri'ounding  the  sun. 

The  question  whether  the  red  protuberances  belong  to  tlie 
moon  or  the  sun  was  settled  during  the  total  eclipse  of  1860, 
wliich  was  observed  in  Spain.  It  was  then  proved  by  meas- 
ures of  their  height  above  the  limb  of  the  moon  that  the  lat- 
ter did  not  carry  them  with  her,  but  passed  over  them.  This 
proved  that  they  were  fixed  relatively  to  the  sun. 

At  the  time  of  this  eclipse  the  spectroscope  was  in  its  in- 
fancy, and  no  one  thought  of  applying  it  to  the  study  of  the 
corona  and  protuberances.  The  next  considerable  eclipse  oc- 
curred eight  years  later,  in  July,  1868,  and  was  visible  in  In- 
dia and  Siam.  The  spectrosco])e  had,  in  the  mean  time,  come 
into  very  general  use,  and  expeditions  were  despatched  from 
several  European  countries  to  India  to  make  an  examination 
of  the  spectra  of  the  objects  in  question.  The  most  success- 
ful observer  was  Janssen,  of  France,  who  took  an  elevated 
position  in  the  interior,  where  the  air  was  remarkably  clear. 
When,  on  the  eventful  day,  the  last  ray  of  sunlight  was  cut 
off  by  the  advancing  moon,  an  enormous  protuberance  showed 
itself,  rising  to  a  height  of  many  thousand  miles  above  the  sur- 
face of  the  sun.  The  spe(;troscope  was  promptly  turned  upon 
it,  and  the  practised  eye  of  the  observer  saw  in  a  moment  that 
the  spectrum  consisted  of  the  bright  lines  due  to  glowing  hy- 
drogen. The  protuberance,  therefore,  did  not  consist  of  any 
substance  shining  merely  by  reflected  sunlight,  but  of  an  im- 
mense mass  of  hydrogen  gas,  so  liot  as  to  shine  by  its  own 
light.  The  theory  of  the  cloud -like  nature  of  the  j)rotuber- 
ances  was  overthrown  in  a  moment. 


THE  SUN'S  SUJiliOUNDIXGS.  '255 

This  observation  marks  the  eomrrencement  of  a  new  era  in 
solar  physics,  which,  by  a  singular  coincidence,  was  inaugu- 
rated independently  by  another  observer.  As  Janssen  looked 
at  the  lines  which  he  was  the  first  of  men  to  see,  it  occurred 
to  him  that  they  were  bright  enough  to  be  seen  after  the  total 
phase  of  the  eclipse  had  passed.  He  therefore  determined  to 
watch  tliem,  and  iind  how  long  he  could  follow  them,  lie 
kept  sight  of  them,  not  only  after  the  total  phase  had  passed, 
but  after  the  eclipse  was  entirely  over.  In  fact,  he  found  that 
with  a  sufficiently  powerful  spectroscope,  he  could  see  the 
spectral  lines  of  the  protuberances  at  any  time  when  the  air 
was  perfectly  clear,  so  that  the  varying  forms  of  these  remark- 
able ol)jects  which  had  hitherto  been  seen  only  during  the 
rare  moments  of  a  total  eclipse  could  be  made  a  subject  of 
regular  observation. 

But  this  great  discovery  was  made  in  England,  independ- 
ently of  the  eclipse,  by  Mr.  J.  Norman  Lockyer.  This  gen- 
tleman was  an  active  student  of  the  subject  of  spectroscopy; 
and  it  had  occurred  to  him  that  the  matter  composing  these 
protuberances,  being  so  near  the  surface  of  the  sun,  must  be 
liot  enough,  not  only  to  shine  by  its  own  light,  but  to  be  quite 
vaporized,  and,  if  so,  its  spectrum  nn'ght  be  seen  by  means  of 
the  spectroscope.  Finding  that  the  instrument  he  possessed 
would  show  nothing,  he  ordered  a  more  powerful  one.  But 
its  constiMiction  was  attended  with  so  much  delay  that  it  was 
not  ready  till  October,  18G8.  On  the  20th  of  that  month,  he 
pointed  it  upon  the  margin  of  the  sun,  and  found  three  bi'ight 
lines  in  the  spectnnn,  two  of  which  belonged  to  hydrogen. 
Thus  was  realized  an  idoa  which  he  had  formed  two  years  be- 
fore, but  which  he  was  prevented  from  carrying  out  by  the 
want  of  a  suitable  instrument.  His  success  was  immediately 
communicated  to  the  French  Academy  of  Sciences,  the  news 
reaching  that  body  on  the  very  day  that  word  was  received 
from  Janssen,  in  India,  that  he  had  also  solved  the  same  prob- 
lem. 

Following  up  his  researches,  Mr.  Lockyer  found  tliat  the 
protuberances  arose  from  a  narrow  envelope  surrounding  the 


256 


THE  SOLAR   SYSTEM. 


* 

^  ■" .  r  ' 

* 

.'^-                 I.       ,■     i 

'  ^'^^^A  ' 

^^   -'Sk 

.^v;.   ' '''^'^^^M^ ''ffiki 

■\           ■■■'  '    Hii       ■! 

i 

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.^'•^^^''Oa^^'                  "  ■ 

■>   :■■■/''..■ 

,jf '■ ' 

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1^ 

Fio.  69. — Speciineus  of  nolar  protuheiaiices,  as  drawn  by  Secchi.    The  brif,'lu  base  in  each 
flijure  represents  the  chromosphere  from  which  the  red  flames  rise. 

whole  surface  of  tlie  snn,  being,  in  fact,  merely  elevated  por- 
tions of  tliis  envelope:  tliat  is  to  say,  the  sun  is  surrounded 
by  an  atmosphere  composed  principally  of  hydrogen  gas,  por- 
tions of  which  are  here  and  there  thrown  u^j  in  tlie  form  of 


THE  SUN'S  SUHROUXDINGS.  257 

enormous  tongues  of  flame,  which,  however,  can  never  be  seen 
except  with  the  8})ectrosco[)e,  or  durin<^  total  eclij>ses.  To  this 
atmosphere  Mr.  Lockyer  gave  the  name  of  the  chromosphere. 
It  had  previously  been  seen  and  recognized  by  several  observ- 
ers during  total  eclipses,  but  nothing  had  been  known  respect- 
ing its  nature. 

The  researches  which  we  have  described  threw  no  light  on 
the  question  of  the  corona,  an  object  which  seemed  to  have 
been  almost  lost  sight  of  in  the  excitement  caused  by  the  dis- 
covery of  the  gaseous  nature  of  the  protuberances.  Happily, 
only  a  year  later,  on  August  7th,  1860,  a  total  eclipse  was  visi- 
ble in  the  United  States.  The  shadow  of  the  moon  passed 
down  the  coast  of  Alaska,  <^hen  entered  into  the  interior,  pass- 
ing over  the  south-west  ])ortion  of  British  America,  entered 
the  United  States  in  the  Territory  of  Nebraska, and  passed  over 
Iowa,  Illinois,  Kentucky,  South-western  Virginia,  and  North 
Carolina.  This  eclipse  was  observed  very  extensively  by 
American  astronomers,  Professor  Ilarkness,  of  the  Naval  Ob- 
servatory, and  Professor  Young,  of  Dartmouth  College,  devot- 
ing especial  attention  to  the  spectroscopic  observations.  These 
observers  found  that  the  corona  gave  a  very  faint,  continuous 
spectrum  crossed  by  a  single  bright-green  line,  M'hicli  was  also 
seen  in  the  spectrum  of  the  protuberances.  This  solitary  line 
was  again  seen  during  the  eclipse  of  December  21st,  ISVO, 
in  the  Mediterranean ;  but  it  has  not  been  certainly  identified 
in  the  spectrum  of  any  known  terrestrial  substance.  There 
are  several  lines  of  iron  in  its  neigliborhood;  but  as  this  line 
stands  alone,  it  does  not  seem  likely  that  it  can  arise  from  the 
vapor  of  iron.  All  vjq  can  say  is,  that  the  substance  Avhich 
gives  this  line,  and  which  seems  to  be  the  only  gaseous  ele- 
ment of  the  corona,  is  unknown,  and  may  possibly  be  some  gas 
much  lighter  than  livdroiJi-en  which  has  not  yet  been  discovered 
on  the  earth. 

Continued  observations  of  the  spectra  of  the  various  gases 
surrounding  the  sun  show  a  much  greater  number  of  lines 
than  have  ever  been  seen  during  total  eclipses.  Mr.  Lockyer 
himself,  by  diligent  observation  extending  over  several  years, 

18 


258  THE  SOLAR  SYSTEM. 

found  over  a  luindred.  But  the  greatest  advance  in  this  re- 
spect was  made  by  Professor  C.  A.  Young.  In  1871  an  astro- 
nomical expedition  was  fitted  out  by  the  Coast  Survey,  for  the 
purpose  of  learning  by  actual  trial  whether  any  great  advan- 
tage would  be  gained  by  establishing  an  ol)servatory  on  the 
most  elevated  point  crossed  by  the  Pacific  Railway.  This 
point  was  Sherman.  The  spectroscopic  part  of  the  expedition 
was  intrusted  to  Professor  Young.  Although  there  was  a 
great  deal  of  cloudy  weather,  yet,  when  the  air  was  clear,  far 
less  light  was  rellected  from  the  sky  surrounding  the  sun  than 
at  lower  altitudes,  which  was  a  great  advantage  in  the  study 
of  the  sun's  surroundings.  Professor  Young  found  no  less 
than  273  bright  lines  which  he  was  able  to  identify  with  cer- 
tainty. The  presence  of  many  known  substances,  especially 
iron,  magnesium,  and  titanium,  is  indicated  by  these  lines; 
but  there  are  also  many  lines  which  are  not  known  to  pertain 
to  any  terrestrial  substance. 

§  6.  Physical  Constitution  of  the  Sun. 

Respecting  the  physical  constitution  of  the  sun,  there  are 
some  points  which  may  be  established  with  more  or  less  cer- 
tainty, but  the  subject  is,  for  the  most  part,  involved  in  doubt 
and  obscurity.  Since  the  properties  of  matter  are  the  same 
everyvdiere,  the  problem  of  the  physical  constitution  of  the 
sun  is  solved  only  when  we  are  able  to  explain  all  solar  phe- 
nomena by  laws  of  physics  which  wo  see  in  operation  around 
us.  The  fact  that  the  physical  laws  operative  on  the  sun  must 
be  at  least  in  agreement  witli  those  in  operation  here,  is  not 
always  remembered  by  those  who  have  speculated  on  the  sub- 
ject. In  stating  what  is  probable,  and  what  is  possible,  in 
the  causes  of  solar  phenomena,  we  shall  begin  on  the  outside, 
and  go  inwards,  because  there  is  less  doubt  about  the  opera- 
tions which  go  on  outside  the  sun  than  about  those  on  his  sur- 
face or  in  the  interior. 

As  we  approach  tlie  sui.,  the  first  material  substance  we 
meet  with  is  the  corona,  rising  to  heights  of  five  or  ten,  per- 
haps even  fifteen,  minutes  above  his  surface,  that  is,  to  a  height 


PHYSICAL   CONSTITUTION  OF  THE  SUN.  259 

of  from  one  to  three  hundred  thousand  miles.  Of  this  ap- 
pendage we  may  say  with  entire  confidence  that  it  cannot  be 
an  atmosphere  in  the  sense  in  which  tliat  word  is  connuonly 
used,  that  is,  a  continuous  mass  of  elastic  gas  held  up  by  its 
own  elasticity.  Of  the  two  reasons  in  favor  of  this  denial,  one 
seems  to  me  almost  conclusive,  the  other  entirely  so.  They 
are  as  follows : 

1.  Gravitation  on  the  sun  is  about  27  times  as  great  as  on 
the  earth,  and  any  gas  is  there  27  times  as  heavy  as  here.  In 
an  atmosphere  each  stratum  is  compressed  by  the  weight  of 
all  the  strata  above  it.  The  result  is,  that  as  we  go  down  by 
successive  equal  steps,  the  density  of  the  atmosphere  increases 
in  geomet'-'cal  progression.  An  atmosphere  of  the  lightest 
known  gas-  -hydrogen — would  double  its  density  every  five  or 
ten  miles,  though  heated  to  as  high  a  temperature  as  is  likely 
to  exist  at  the  height  of  a  hundred  thousand  miles  above  the 
sun's  surface.  But  there  is  no  approximation  to  such  a  rapid 
increase  in  the  density  of  the  corona  as  we  go  downwards.  If 
we  suppose  the  corona  to  be  such  an  atmosphere,  we  must 
suppose  it  to  be  hundreds  of  times  lighter  than  hydrogen. 

2.  The  great  comet  of  1843  passed  within  three  or  four 
minutes  of  the  surface  of  the  sun,  and  therefore  directly 
through  the  midst  of  the  corona.  At  the  time  of  nearest  ap- 
proach its  velocity  was  350  miles  per  second,  and  it  went  with 
nearly  this  velocity  through  at  least  300,000  miles  of  corona, 
coming  out  without  having  suffered  any  visible  damage  or 
retardation.  To  form  an  idea  what  would  have  become  of 
it  had  it  encountered  the  rarest  conceivable  atmosphere,  we 
have  only  to  reflect  that  shooting-stars  are  instantly  and  com- 
pletely vaporized  by  the  heat  caused  by  their  encounter  with 
our  atmosphere  at  heights  of  from  50  to  100  miles;  that  is,  at 
a  height  where  the  atmosphere  entirely  ceases  to  reflect  the 
light  of  the  sun.  The  velocity  of  shooting-stars  is  from  20  to 
40  miles  per  second.  Ilcmembering,  now,  that  resistance  and 
heat  increase  at  least  as  the  square  of  the  velocity,  what  would 
be  the  fate  of  a  body,  or  a  collection  of  bodies  like  a  comet, 
passing  through  several  hundred  thousand  miles  of  the  rarest 


260'  THE  SOLAR  SYSTEM. 

atmosphere  at  a  rate  of  over  300  miles  a  second  ?  And  how 
rare  must  such  an  atmosphere  be  when  the  comet  passes  not 
only  without  destruction,  but  without  losing  any  sensible  ve- 
locity !  Certainly  so  rare  as  to  be  entirely  invisible,  and  inca- 
pable of  producing  any  physical  effect. 

What,  then,  is  the  corona?  Probably  detached  particles 
partially  or  wholly  vaporized  by  the  intense  heat  to  which 
they  are  exposed.  A  mere  dust -particle  in  a  cubic  mile  of 
space  would  shine  intensely  when  exposed  to  such  a  flood  of 
light  as  the  sun  pours  out  on  every  body  in  his  neighborhood. 
The  difficult  question  which  we  meet  is.  How  are  these  parti- 
cles held  up?  To  this  question  only  conjectural  replies  can 
be  given.  That  the  particles  are  not  permanently  held  in  one 
position  is  shown  by  the  fact  that  the  form  of  the  corona  is 
subject  to  great  variations.  In  the  eclipse  of  1869,  Dr.  Gould 
thoui>:ht  he  detected  variations  durino:  the  three  minutes  the 
eclipse  lasted.  The  three  conjectures  that  have  been  formed 
on  the  subject  are : 

1.  That  the  matter  of  the  corona  is  in  what  we  may  call  a 
state  of  projection,  being  constantly  thrown  up  by  the  sun, 
while  each  particle  thus  projected  falls  down  again  according 
to  the  law  of  gravitation.  Tlie  difficulty  we  encounter  here  is 
that  we  must  suppose  velocities  of  projection  rising  as  liigh  as 
200  miles  per  second  constantly  maintained  in  every  region 
of  the  solar  globe. 

2.  That  the  particles  thrown  out  by  the  sun  are  held  up  a 
greater  or  less  time  by  electrical  repulsion.  We  know  that  at- 
mospheric electricity  plays  an  active  part  in  terrestrial  mete- 
orology ;  and  if  electric  action  at  tlie  surface  of  the  sun  is  pro- 
portional to  those  physical  and  chemical  actions  which  we 
li!)d  to  give  rise  to  electrical  phenomena  here  on  the  earth, 
the  development  of  electricity  there  must  be  on  an  enormous 
scale. 

3.  That  the  corona  is  duo  to  clouds  of  minute  meteors  cir- 
culating around  the  sun  in  the  immediate  vicinity  of  that  lu- 
minary. 

As  already  intimated,  none  of  these  explanations  is  much 


PHYSICAL   CONSTITUTION  OF  THE  SUN. 


261 


better  than  a  conjecture,  though  it  is  quite  probable  that  the 
facts  of  the  case  are  divided  somewhere  among  them. 

Next  inside  the  corona  lies  the  chromosphere.  Here  we 
reach  the  true  atmosphere  of  the  sun,  rising  in  general  a  few 
seconds  above  his  sui-fat  3,  but  now  and  then  projected  up- 
wards in  immense  masse.^  which  we  might  call  flame,  if  the 
word  were  not  entii-ely  inadequate  to  convey  any  conception 


Fig,  To. — The  biui,  wiUi  its  chioinosnheie  uml  red  flanu'si.  on  .Inly  'i:u\,  IbTl,  aa  drawu  by 
Secclii.    The  figures  mark  the  flames,  17  iu  number. 

of  the  enormous  scale  on  which  thermal  action  is  there  car- 
ried on.  What  we  call  fire  and  flame  are  results  of  burn- 
ing; but  the  gases  at  the  surface  of  the  sun  are  already  «o 
hot  that  burning  is  not  possible.  Hydrogen  is  the  principal 
material  of  the  u])per  ])art  of  the  chromospliere ;  but,  as  we 
descend,  we  find  the  vapors  of  a  great  number  of  metals,  in- 
cluding iron  and  magnesiutn.  At  the  base,  where  the  metals 
are  most  nun.,  rous,  and  the  density  the  greatest,  occurs  the 
absorption  of  the  solar  rays  which  causes  the  dark  lines  in  the 


262  THE  SOLAR  SYSTEM. 

spectrum  already  described  (p.  225).  This  seems  satisfactori- 
ly proved  by  an  observation  of  Professor  Yonng's  during  the 
eclipse  of  1870,  in  Spain.  At  the  moment  of  disappearance 
of  the  last  rays  of  sunlight,  when  he  had  a  glimpse  of  the 
base  of  the  chromosphere,  he  saw  all  the  spectral  lines  re- 
versed ;  that  is,  they  were  bright  lines  on  a  dark  ground.  The 
vapors  which  absorb  certain  rays  of  the  light  which  passes 
through  them  from  the  sun  then  emitted  those  same  rays 
when  the  sunlight  was  cut  off. 

The  most  astonishing  phenomena  connected  with  the  chro- 
mosphere are  those  outbursts  of  its  matter  which  form  the  pro- 
tuberances. The  latter  are  of  two  classes — the  cloud-like  and 
the  eruptive.  The  first  class  presents  the  appearance  of  clouds 
floating  in  an  atmosphere ;  but  as  no  atmosphere  dense  enougli 
to  sustain  anything  can  possibly  exist  there,  we  find  the  same 
difficulty  in  accounting  for  them  that  we  do  in  accounting  for 
the  suspension  of  the  matter  of  the  corona.  In  fact,  of  the 
three  conjectural  explanations  of  the  corona,  two  are  inadmis- 
sible if  applied  to  the  protuberances,  since  these  cloud-like 
bodies  sometimes  remain  at  rest  too  long  to  bo  supposed  mov- 
ing under  the  influence  of  the  sun's  gravitation.  This  leaves 
the  electrical  explanation  as  the  only  adequate  one  yet  brought 
forward.  The  eruptive  protuberances  seem  to  be  due  to  the 
projection  of  hydrogen  and  magnesium  vapor  from  the  region 
of  the  chromosphere  with  velocities  which  sometimes  rise  to 
150  miles  a  second.  The  eruption  may  continue  for  hours,  or 
even  days,  the  vapor  spreading  out  into  great  masses  thousands 
of  miles  in  extent,  and  then  falling  back  on  the  chromosphere. 

Is  it  possible  to  present  in  language  any  adequate  idea  of 
the  scale  on  which  natural  operations  are  here  carried  on  ?  If 
we  call  the  chromosphere  an  ocean  of  fire,  we  must  remember 
that  it  is  an  ocean  hotter  tlian  the  fiercest  furnace,  and  as  deep 
as  the  Atlantic  is  broad.  If  we  call  its  movements  hurricanes, 
we  must  remember  that  our  hurricanes  blow  only  about  a  hun- 
dred miles  an  hour,  while  those  of  the  chromosphere  blow  as 
far  in  a  single  second.  They  are  such  hurricanes  as,  "  coming 
down  upon  us  from  the  north,  would,  in  thirty  seconds  after 


PHYSICAL   CONSTITUTION  OF  THE  SUN.  263 

they  had  crossed  the  St.  Lawrence,  be  in  the  Gulf  of  Mexico, 
carrying  with  them  the  wliole  surface  of  the  continent  in  a 
mass,  not  simply  of  ruin,  but  of  glowing  vapor,  in  which  the 
vapors  arising  from  the  dissolution  of  the  materials  composing 
the  cities  of  Boston,  New  York,  and  Chicago  would  be  mixed 
in  a  single  indistinguishable  cloud."  Wlien  we  speak  of  erup- 
tions, we  call  to  mind  \'  esuvius  burying  the  surrounding  cities 
in  lava ;  but  the  solar  eruptions,  thrown  fifty  thousand  miles 
high,  would  ingulf  the  whole  eartli,  and  dissolve  every  organ- 
ized being  on  its  surface  in  a  moment.  When  the  mediaeval 
poets  sung, 

"  Dies  ira;,  dies  ilia 
Solvet  saeclum  in  favilla," 

they  gave  rein  to  their  wildest  imagination,  without  reaJiing 
any  conception  of  the  magnitude  or  fierceness  of  the  flames 
around  the  sun. 

Of  the  corona  and  chromosphere  the  telescope  ordinarily 
shows  us  nothing.  They  are  visible  only  during  total  eclipses, 
or  by  the  aid  of  the  spectroscope.  All  w^e  see  with  the  eye  or 
the  telescope  is  the  shining  surface  of  the  sun  called  the  pho- 
tosphere, on  which  the  chromosphere  rests.  It  is  tl  i?  vhich 
radiates  both  the  light  and  the  heat  which  reach  us.  The 
opinions  of  students  respecting  the  constitution  of  the  photo- 
sphere are  so  different  that  it  is  hardly  possible  to  express  any 
views  that  will  not  be  challenged  in  some  quarter.  Although 
a  contrary  opinion  is  held  by  many,  we  may  venture  to  say 
that  the  rays  of  light  and  lieat  seem  to  come,  not  from  a 
gas,  but  from  solid  matter.  This  is  indicated  by  the  fact  that 
their  spectrum  is  continuous,  and  also  by  the  intensity  of  the 
light,  which  far  exceeds  any  that  a  gas  has  ever  been  made 
to  give  forth.  It  does  not  follow  from  this  that  the  photo- 
sphere is  a  continuous  solid  or  crust,  since  floating  particles  of 
solid  matter  will  shine  in  the  same  way.  The  general  opinion 
has  been  that  the  photosphere  is  of  a  cloud-like  nature ;  that 
is,  of  minute  particles  floating  in  an  atmosphere  of  heated  gases. 
That  it  is  not  continuously  solid  like  our  earth  seemed  to  be 
fully  shown  by  the  variations  and  motions  of  the  spots,  which 


264  THE  SOLAR  SYSTEM. 

have  every  appearance  of  going  on  in  a  fluid  or  gas.  Indeed, 
of  late,  some  of  the  most  eminent  physicists  regard  it  as  pure- 
ly gaseous,  the  pressure  making  it  shine  like  a  solid. 

But  this  theory  is  attended  with  a  difficulty  which  has  not 
l)een  sufficiently  considered.  The  photosphere  is  in  striking 
contrast  to  the  gaseous  chromosphere,  in  being  subject  to  no 
sensible  changes  of  level.  If  it  were  gaseous,  as  supposed, 
the  solid  particles  having  no  connection  with  each  other,  we 
should  expect  those  violent  eruptions  which  throw  up  the  pro- 
tuberances to  carry  up  portions  of  it,  so  that  it  would  now  and 
then  present  an  irregular  and  jagged  outline,  as  the  chromo- 
sphere does.  But  the  most  retined  observations  have  never 
shown  it  to  be  subject  to  the  slightest  change  of  level,  or  devi- 
ation from  perfect  rotundity,  except  in  tlie  region  of  the  spots, 
where  its  continuity  seems  to  be  broken  by  immense  chasm- 
like openings. 

The  serene  immobility  of  the  photosphere,  under  such  vio- 
lent actions  around  it  as  w^e  have  described,  lends  some  color 
to  the  supposition  that  it  is  a  solid  crust  which  forms  around 
the  glowing  interior  of  the  sun,  or,  at  least,  that  it  is  composed 
of  a  comparatively  dense  fluid  resting  upon  such  a  crust.  The 
latter  is  tlie  view  of  Zollner,  who  considers  some  sort  of  an 
envelope  between  the  exterior  and  the  interior  of  the  sun  ab- 
solutely necessary  to  account  for  the  eruptive  protuberances. 
He  places  this  solid  envelope  three  or  four  thousand  miles  be- 
low the  surface  of  the  photosphere. 

Inside  the  photosphere  we  have  the  enormous  interior 
globe,  860,000  miles  in  diameter.  The  best-sustained  theory 
of  the  interior  is  the  startling  one  that  it  is  neither  solid  nor 
liquid,  but  gaseous;  so  that  our  great  luminary  is  nothing 
more  than  an  immense  bubble.  The  pressure  upon  the  inte- 
rior portions  of  this  mass  is  such  as  to  reduce  it  to  nearly  the 
density  of  a  liquid ;  while  the  temperature  is  so  high  as  to 
keep  the  substances  in  a  state  which  is  between  the  liquid  and 
the  gaseous,  and  in  which  no  chemical  action  is  possible.  The 
strong  point  in  support  of  this  gaseous  theory  of  the  sun's  in- 
terior isj  that  it  is  the  only  one  which  explains  how  the  sun's 


VIEWS  ON  TEE  PHYSICAL  COXSTITUTIOX  OF  TEE  SUX.   266 

light  and  heat  are  kept  up.  How  it  does  this  will  be  shown 
in  treating  of  the  laws  whi(;h  gONcrn  the  secular  changes  of 
the  universe  at  large. 

§  7.  Views  of  Distinguished  Students  of  the  Sun  on  the  Subject  of 

its  Physical  Constitution. 

The  progress  of  our  knowledge  of  the  sun  during  the  past 
ten  yeai-s  has  been  so  rapid  that  only  those  can  completely  fol- 
low it  who  make  it  the  principal  business  of  their  lives.  For 
the  same  reason,  the  views  respecting  the  sun  entertained  by 
those  who  are  engaged  in  studying  it  must  be  modified  and 
extended  from  time  to  time.  The  interest  which  necessarily 
attaches  to  the  physical  source  of  all  life  and  motion  on  our 
globe  renders  the  author  desirous  of  presenting  these  views  to 
his  readers  in  their  latest  form  ;  and,  through  the  kindness  of 
several  of  the  most  eminent  i'lvestigators  of  solar  physics  now 
living,  he  is  enabled  to  gratify  that  desire.  The  following 
statements  are  presented  in  the  language  of  their  respective 
authors,  except  that,  in  the  case  of  Messrs.  Secchi  and  Faye, 
they  are  translated  from  the  French  for  the  convenience  of 
the  English  reader.  It  will  be  noticed  that  in  some  minor 
points  they  differ  from  each  other,  as  well  as  from  those  which 
the  author  has  expressed  in  the  preceding  section.  Such  dif- 
ferences are  unavoidable  in  the  investigation  of  so  difli"Mlt  a 
subject. 

Views  of  the  Rev.  Father  Secchi. — "  For  me,  as  for  every  one 
else,  the  sun  is  an  incandescent  body,  raised  to  an  enormous 
temperature,  in  which  the  substances  known  to  our  chennsts 
and  physicists,  as  well  as  several  other  substances  still  unknown, 
are  in  a  state  of  vapor,  heated  to  such  a  degree  that  its  spec- 
trum is  continuous,  either  on  account  of  the  pressure  to  which 
the  vapor  is  subjected,  or  of  its  higli  temperature.  This  i'^can- 
descent  mass  is  what  constitutes  the  photosphere.  Its  limit  is 
defined,  as  in  the  case  of  incandescent  gases  in  general,  by  the 
temperature  to  which  the  exterior  layer  is  reduced  by  its  free 
radiation  in  space,  together  with  the  force  of  gravity  exert- 
ed by  the  body.     The  ])hotosphere  presents  itself  as  composed 


266  THE  SOLAR  SYSTEM. 

of  small,  brilliant  granulations,  separated  by  a  dark  net-work. 
These  granulations  are  only  the  summits  of  the  flames  which 
constitute  them,  and  which  rise  above  tliO  lower  absorbing 
layer,  which  forms  the  net- work,  as  we  shall  soon  more  clearly 
see. 

"Above  the  photospheric  layer  lies  an  atmosphere  of  a  very 
complex  nature.  At  its  base  are  the  heovy  metallic  vapors, 
at  a  temperature  which,  being  less  elevated,  no  longer  permits 
the  emission  of  light  with  a  continuous  spectrum,  although  it 
is  sufficient  to  give  direct  spectra  with  brilliant  lines,  which 
may  be  observed,  during  total  eclipses  of  the  sun,  at  its  limb. 
This  layer  is  extremely  thin,  having  a  depth  of  only  one  or 
tv/o  seconds  of  arc.  According  to  the  law  of  absorption  laid 
down  by  Kirchhoff,  these  vapors  absorb  the  rays  of  the  spec- 
trum from  the  light  of  the  photosphere  which  passes  through 
them,  thus  giving  rise  to  the  breaks  known  as  the  Fraunhofer 
dark  lines  of  the  solar  spectrum.  These  vapors  are  mixed 
with  an  enormous  quantity  of  hydrogen.  This  gas  is  present 
in  such  a  quantity  that  it  rises  considerably  above  the  other 
layer,  and  forms  an  envelope  rising  to  a  height  of  from  ten 
to  sixteen  seconds,  or  even  more,  which  constitutes  what  we 
call  the  chromosphere.  This  hydrogen  is  always  mixed  with 
another  substance, provisionally  called  ]ielium,vf\\\c\i  forms  the 
yellow  line  D^  of  the  spectrum  of  the  protuberances,  and  with 
another  still  rarer  substance,  which  gives  the  green  line  1474 
K.  This  last  substance  rises  to  a  much  greater  elevation  than 
the  hydrogen ;  but  it  is  not  so  easily  seen  in  the  full  sun  as 
the  latter.  Probably  there  is  some  other  substance  not  yet 
well  determined.  Thus,  the  substances  which  compose  this 
solar  envelope  appear  to  be  arranged  in  the  order  of  their 
density ;  but  still  without  any  well-defined  separation,  the  dif- 
fusion of  the  gases  producing  a  constant  mixture. 

"  This  atmosphere  becomes  visible  in  total  eclipses  in  the 
form  of  the  corona.  It  is  very  difficult  to  fix  its  absolute 
heiglit.  The  eclipses  prove  that  it  may  reach  to  a  height 
equal  to  the  solar  diameter  in  its  highest  portions. 

"  No  doubt  it  extends  yet  farther,  and  it  may  well  be  con- 


VIEWS  ON  THE  PHYSICAL  CONSTITUTION  OF  THE  SUN    267 

nected  with  the  zodiacal  light.  The  visible  layer  of  this  at- 
mosphere is  not  spherical ;  it  is  higher  in  middle  latitudes, 
near  forty-live  degrees,  than  at  the  equator.  It  is  still  more 
depressed  at  the  poles.  At  the  base  of  the  chromosphere, 
the  hydrogen  has  the  shape  of  small  tlames  composed  of  very 
thin,  close  tilaments  which  seem  to  correspond  to  tlie  granu- 
lations of  the  photosphere.  During  periods  of  traufpiilUty 
the  direction  of  these  filaments  is  perpendicular  to  the  solar 
surface ;  but  during  periods  of  agitation  they  are  generally 
more  or  less  inclined,  and  often  directed  systematically  tow- 
ards the  poles. 

"  The  body  of  the  sun  is  never  in  a  state  of  absolute  repose. 
The  various  substances  coming  together  in  the  interior  of  the 
body  tend  to  combine,  in  conse(|uence  of  their  affinity,  and 
necessarily  produce  agitations  and  interior  movements  of  every 
kind  and  of  great  intensity.  Hence  the  numerous  crises  which 
show  themselves  at  the  surface  through  the  elevation  of  the 
lower  strata  of  the  r.tmosphere  by  eruptions,  and  often  by  act- 
ual explosions.  Then  the  lower  metallic  vapors  are  projected 
to  considerable  heights,  hydi'ogen  especially,  at  an  elevation 
visible  in  the  spectroscope  (in  full  sunlight)  of  one-fourth  the 
solar  diameter.  These  masses  of  hydrogen,  leaving  the  pho- 
tosphere at  a  temperature  higher  than  that  of  the  atmosphere, 
rise  to  the  superior  regions  of  the  latter,  remaining  suspend- 
ed, diffusing  themselves  at  considerable  elevations,  and  form- 
ing what  are  called  the  prominences  or  protuberances.  The 
structure  of  the  hydrogenous  protuberances  is  entirely  simi- 
lar to  that  of  fluid  veins  raising  themselves  from  denser  la3'ers, 
and  diffusing  in  the  more  rare  ones :  but  their  extreme  varia- 
bility, even  at  the  base,  and  the  rapid  changes  of  the  place  of 
exit  and  diffusion,  prove  that  they  do  not  pass  througli  any 
oriflce  in  a  solid  resisting  layer. 

"  These  eruptions  are  often  mixed  with  columns  of  metallic 
vapors  of  greater  density,  which  do  not  attain  the  elevation 
of  the  hydrogen,  and  of  which  the  nature  can  be  recognized 
by  the  aid  of  the  spectroscope :  occasionally  we  see  them  fall- 
ing back  on  the  sun  in  the  form  of  parabolic  jets.     The  most 


268  THE  SOLAR  SYSTEM. 

common  substances  arc  sodium,  magnesium,  iron,  calcium,  etc. 
■ — indeed,  the  same  substances  which  are  seen  to  form  the  low, 
absorbing  layer  of  tlie  solar  atmos))here,  and  which  by  their 
absorption  produce  the  Fi'aunhofer  lines.  A  rigorous  and  in- 
evitable conse(|uence  of  these  conditions  is  tlie  fact  that  when 
the  mass  thus  elevated  is  carried  by  tiie  rotation  of  the  sun 
between  the  photosi)here  and  the  eye  of  the  observer,  the  ab- 
sorption becomes  ver}'  sensii)le.  and  produces  a  dark  spot  on 
the  photosphere  itself.  Tlie  metallic  absor[)tion  lines  are 
then  really  wider  and  more  diffused  in  this  region  ;  and  if 
the  elevated  mass  is  liigli  and  dense  enough,  we  can  even  see 
the  re-reversal  of  the  lines  already  reversed ;  that  is  to  say, 
we  can  see  the  bright  lines  of  the  sub'-^ance  itself  on  the  back- 
ground of  the  spot.  Tiiis  often  hap])ens  for  hydrogen,  which 
rises  to  a  great  height,  and  also  with  sodium  and  magnesium, 
whicli  metals  have  the  rarest  vapors.  Here,  then,  we  have  tlie 
origin  of  the  solar  spots.  They  are  formed  by  masses  of  ab- 
sorbing vapors  which,  brought  out  from  the  interior  of  the  sun, 
and  interposed  between  the  ])hotosphere  and  the  eye  of  the  ob- 
server, prevent  a  large  piart  of  the  light  from  reaching  our  eyes. 
"But  these  vapors  are  heavier  than  the  surrounding  n.ass 
into  which  they  have  been  thrown.  They  therefore  fall  by 
their  own  weight,  and,  tending  to  sink  into  the  pliotosphere, 
produce  in  it  a  sort  of  cavity  or  basin  filled  with  a  darker  and 
moi-e  al)sorbing  mass.  Hence  the  aspect  of  a  cavity  recognized 
in  the  spots.  If  the  eruption  is  instantaneous,  or  of  very  short 
duration,  this  vaporous  mass,  fallen  back  on  the  i)hotosj)here, 
soon  becomes  incandescent,  reheated,  and  dissolved,  and  the 
spot  I'apidly  disa})pears;  but  the  interior  crises  of  the  body  of 
the  sun  may  be  continued  a  long  time;  and  the  eruption  may 
maintain  itself  in  the  same  place  during  two  or  more  rotations 
of  the  sun.  Hence  the  persistence  of  the  spots;  for  the  cloud 
can  continue  to  form  so  long  and  so  fast  as  the  photosphere 
dissolves  it,  as  happeub  with  the  jets  of  vapor  from  our  vol- 
canoes. The  eru[)tions,  when  about  to  ternn'nate,  may  be  re- 
vived and  reproduced  several  times  near  the  same  place,  and 
give  rise  to  spots  very  variable  in  form  and  position. 


rJEJVS  ON  THE  PUYSICAL  CONSTITUTION  OF  THE  SUN.   2G9 

"  The  spots  arc  formed  of  a  central  region,  called  the  nu- 
cleus, or  umbra,  and  of  a  surrounding  part  less  dark,  called 
the  penumbra.  The  latter  is  really  formed  of  thin  dark  veils, 
and  of  iihiments  or  currents  of  photospheric  matter  which 
tend  to  encroach  upon  the  dark  mass.  These  currents  have 
the  form  of  tongues,  often  composed  of  globular  masses  )•  k- 
ing  like  strings  of  beads  or  willow  leaves,  and  evidently  are 
only  the  grains  of  the  photosphere  precipitating  themselves 
towards  the  centre  of  the  spot,  and  sometimes  crossing  it  like 
a  bridge. 


Pi'i.  71 — IllusUatiiig  Sfcchi'K  theory  ol' solar  spots. 

"  In  each  spot  we  must  distinguish  tln-ec  periods  of  exist- 
ence :  the  first,  of  formation ;  tlie  second,  of  rest ;  the  third, 
of  extinction.  In  the  first,  tiie  photospheric  mass  is  raised 
and  distorted  by  a  great  agitation,  often  in  tlie  nature  of  a 
vortex,  which  elevates  it  all  around  the  flowing  streams,  and 
forms  irregular  elevations,  either  without  penumbra  or  with  a 
very  irregular  one.  These  irregular  movements  defy  descrip- 
tion :  their  velocities  are  enormous,  and  the  agitated  region 


'270  THE  SOLAR  SYSTEM. 

extends  itself  over  several  square  degrees;  but  tliis  upturn- 
ing soon  comes  to  an  end,  and  the  agitation  slowly  subsides, 
and  is  succeeded  by  calm.  In  the  second  period,  the  agi- 
tated and  elevated  mass  falls  back  again,  and  tends  to  com- 
bine in  masses  more  or  less  circular,  and  to  sink  by  its  weight 
into  the  surface  of  the  photosphere.  Hence  the  depressed 
form  of  the  photosphere,  resembling  a  funnel,  and  the  numer- 
ous currents  which  come  from  each  point  of  the  circumference 
to  rush  upon  this  obs(nn-e  mass ;  but  at  the  same  time  the  con- 
trast between  it  and  the  substance  issuing  still  persists.  The 
spot  takes  a  nearly  stable  and  circular  form,  a  contrast  which 
may  last  a  long  time — so  long,  in  fact,  as  the  interior  actions  of 
the  solar  globe  furnish  new  materials.  At  length,  the  latter 
ceasing,  the  eruptive  action  languishes  and  is  exhausted,  and 
the  absorbing  mass  invaded  on  all  sides  by  the  photosphere  is 
dissolved  and  absorbed,  and  the  spot  disappears. 

"  The  existence  of  these  three  phases  is  established  by  the 
comparative  study  of  the  spots  and  eruptions.  When  a  spot 
is  on  the  sun's  border  during  its  first  period,  although  the 
dark  region  is  invisible,  its  position  is  indicated  by  eruptions 
of  metallic  vapors,  if  the  spot  be  considerable.  On  the  dark- 
est ones  the  vapors  of  sodium,  iron,  and  magnesium  are  seen 
in  the  greatest  quantity,  and  raised  to  great  heights.  A  calm 
and  circular  spot  is  crowned  by  beautiful  faculre  and  jets  of 
hydrogen  and  metallic  vapors,  very  low,  though  quite  brilliant. 
A  spot  which  is  on  the  point  of  closing  up  has  no  metallic 
jets,  arid  at  the  utmost  only  a  few  small  jets  of  hydrogen,  and 
a  more  agitated  and  elevated  chromosphere.  Besides,  obser- 
vation teaches  that  the  eruptions  in  general  accompany  the 
spots,  and  that  they  are  defi^'^ient  at  times  when  the  spots  are 
wanting.  Thus  the  solar  activity  is  measured  by  the  double 
activity  of  eruptions  and  spots  which  have  a  common  source, 
and  the  spots  are  really  only  a  secondary  phenomenon,  de- 
pending upon  the  eruptions  and  the  more  or  less  absorbing 
quality  of  the  materials  :  if  the  erupted  materials  were  not 
absorbent,  we  could  see  no  spots  at  all. 

"  The  eruptions  composed  simply  of  hydrogen  do  not  pro- 


VIEWS  OJV  THE  PHYSICAL  CONSTITUTION  OF  THE  SUN.   271 

dnce  spots;  thus  they  are  seen  on  all  points  of  the  disk,  while 
the  spots  are  limited  to  the  tropical  zones,  where  alone  the 
metallic  eruptions  appear.  The  eruptions  of  simple  hydrogen 
give  rise  to  tlie  facnlne.  The  greater  brilliancy  of  the  facula3 
is  due  to  two  causes:  the  first  is,  the  elevation  of  the  photo- 
sphere above  the  absorbing  stratum  of  vapor  which  is  very 
thin  (only  one  or  two  seconds  of  arc,  as  we  have  before  said) ; 
this  elevated  region  thus  escapes  the  absorption  of  the  lower 
stratum,  and  appears  more  brilliant.  The  other  cause  may  be 
that  the  hydrogen,  in  coming  out,  displaces  the  absorbing 
stratum,  and,  taking  the  place  of  the  metallic  vapors,  permits 
a  better  view  of  ^lie  light  of  the  photosphere  itself. 

"Thus,  in  conci  ision,  the  spots  are  a  seconrlary  phenomenon, 
but,  nevertneless,  inform  us  of  the  violent  crises  whicli  pre- 
vail in  the  interior  of  the  radiant  globe.  The  frequency  of 
the  spots  corresponding  to  the  frequency  of  eruptions,  the  two 
phenomena,  taken  in  connection,  are  the  mark  of  solar  activ- 
ity. The  spots  occupy  the  zones  on  each  side  of  the  solar 
equator,  and  rarely  pass  beyond  the  parallel  of  thirty  degrees. 
One  or  two  seen  at  forty-five  degrees  are  exceptions.  That 
parallel  is  therefore  the  limit  of  greatest  activity  of  the  body. 
It  is  remarkable  that  the  parallels  of  thirty  degrees  divide  the 
hemisph.eres  into  two  sectors  of  equal  volume.  Beyond  these 
parallels  we  see  facula%  but  not  true  spots — or,  at  riiost,  orily 
veiled  spots  indicative  of  a  very  feeble  metallic  eruption. 

"  Such  a  fluid  mass,  in  which  the  parts  are  exposed  to  very 
different  temperatures,  could  not  subsist  without  an  interior 
circulation.  We  do  not  yet  know  its  laws ;  but  the  following 
facts  are  well  enough  estal)lished :  the  zones  of  spots  are  not 
fixed,  but  have  a  progressive  motion  from  the  equator  towards 
the  poles.  The  spots,  arrived  at  a  certain  high  latitude,  cease 
to  aj)pear,  but  after  some  time  reappear  at  lower  latitudes, 
and  afterwards  go  on  anew.  Between  these  pliases  of  dis- 
placement there  is  commonly  a  minimum  of  spots.  During 
periods  of  activity  the  protuberances  have  a  dominant  direc- 
tion towards  the  pole,  as  also  the  flames  of  the  chromosphere. 
Tliis  indicates  a  general  movement  of  the  photosphere  from 


272  THE  SOLAR  SYSTEM. 

the  equator  to  tlie  poles.  This  movement  is  supported  by  the 
displacement  of  the  zones  of  eruption  and  of  the  protuber- 
ances, which  always  seem  to  move  towards  the  poles. 

"  Besides  this  movement  in  latitude,  the  photosphere  has 
also  a  movement  in  longitude,  which  is  greatest  at  the  equa- 
tor. Thus  the  time  of  rotation  of  the  body  is  different  upon 
differetit  parallels,  the  minimum  being  at  the  equator.  These 
phenomena  lead  to  the  conclusion  that  the  entire  mass  is  af- 
fected with  a  vortical  motion  which  sets  from  the  equator 
towards  the  poles,  in  a  direction  oblique  to  the  meridians. 
The  theory  of  these  movements  is  still  to  be  elaborated,  and 
is,  no  doubt,  connected  with  the  primitive  mode  in  which  the 
sun  was  formed. 

"  The  activity  of  the  body  is  subject  to  considerable  fluctu- 
ations :  the  best  established  period  is  one  of  eleven  and  one- 
third  years,  but  the  activity  increases  more  rapidly  than  it  di- 
minishes— it  increases  about  four  vears,  and  diminishes  about 
seven.  This  activity  is  connected  with  the  phenomena  of  ter- 
restrial magnetism,  but  we  cannot  say  in  what  way.  We  may 
suppose  a  direct  electro-magnetic  influence  of  the  sun  upon 
our  globe,  or  an  indirect  influence  due  to  the  thermal  action 
of  the  sun,  which  reacts  npon  its  magnetism.  It  is,  indeed, 
very  natural  to  suppose  that  the  ethereal  mass  which  fills  the 
spaces  of  our  planetary  system  may  be  greatly  altered  and 
modified  by  the  activity  of  the  central  body.  But,  whatever 
may  be  the  cause  of  these  changes  of  activity,  we  are  com- 
pletely ignorant  of  them.  The  action  of  the  planets  has  been 
proposed  as  plausible,  but  it  is  far  from  being  satisfactory. 
The  true  explanation  is  reserved  for  the  science  which  shall 
reveal  the  nature  of  the  connection  which  unites  heat  to  elec- 
tricity, to  magnetism,  and  to  the  cause  of  gravity. 

"  Of  the  interior  of  the  sun  we  have  no  certain  information. 
The  superficial  temperature  is  so  great,  notwithstanding  the 
continual  loss  of  heat  which  it  suffers,  that  we  cannot  8U[)pose 
it  less  in  the  interior ;  and,  consequently,  no  solid  layer  can  ex- 
ist there,  except  perhaps  at  depths  where  the  pressure  due  to 
gravity  equals  or  surpasses  the  molecular  dilatation  produced 


VIEWS  ON  THE  PHYSICAL  CONSTITUTION  OF  THE  SUN.   273 

by  temperature.  However  it  may  be,  the  layer  accessible  to 
the  exploration  of  our  instruments  is,  no  doubt,  lluid  and  gase- 
ous, and  we  can  thus  ex[)lain  the  variations  of  the  solar  diam- 
eter established  by  certain  astronomers.  Notwithstanding  these 
small  fluctuations,  the  radiation  of  the  body  into  its  planetary 
system  is  nearly  constant  during  widely  sepaiated  periods,  and 
especially  is  it  so  during  the  historic  period.  This  constancy- 
is  due  to  several  causes :  first,  to  the  enormous  mass  of  the 
body,  which  can  be  cooled  only  very  slowly,  owing  to  its  very 
high  temperature ;  second,  to  the  contraction  of  the  mass, 
which  accompanies  the  condensation  consequent  upon  the  loss 
of  heat ;  third,  to  the  emission  of  the  heat  of  dissociation  due 
to  the  production  of  chemical  actions  which  may  take  place 
in  the  total  mass. 

"  The  origin  of  this  heat  is  to  be  found  in  the  force  of  grav- 
ity ;  for  it  is  well  proved  that  the  solar  mass,  by  contracting 
from  the  limits  of  the  planetary  system  to  its  present  volume, 
would  produce,  not  only  its  actual  temperature,  but  one  sev- 
eral times  greater.  As  to  the  absolute  value  of  this  tempera- 
ture, we  cannot  fix  it  with  certainty.  Science  not  yet  having 
determined  the  relation  which  exists  between  molecular  liv- 
ing force  {vis  viva)  and  the  intensity  of  radiation  to  a  distance 
(which  last  is  the  only  datum  given  by  observation),  we  find 
ourselves  in  a  state  of  ])ainf ul  uncertainty.  Nevertheless,  this 
temperature  must  be  several  million  degrees  of  our  thermom- 
eter, and  capable  of  maintaining  all  known  substances  in  a 
state  of  vapor. 

"  Rome,  February  lltli,  1877." 

Views  of  A[.  Faye. — "  In  studying  without  any  prepossession 
the  movements  of  the  spots,  we  find,  with  Mr.  Carrington,  that 
there  exists  a  simple  relation  between  their  latitude  and  their 
angular  velocity.  Nevertheless,  this  law  does  not  suffice  to 
represent  the  observations  with  the  exactitude  which  they  ad- 
mit of.  It  is  still  necessary  to  take  account  by  calculation  of  a 
parallax  of  depth  which  I  estimate  at  -j^  of  the  radius  of  the 
sun,  and  of  certain  oscillations  of  very  small  extent,  and  of 
long  period,  which  the  spots  undergo  perpendicular  to  their 

19 


274  THE  SOLAR  SYSTEM. 

parallels.  Then  the  observations  are  represented  with  gruat 
precision,  from  which  I  conclude  that  Ave  have  to  deal  with  a 
quite  simple  niGclianical  phenomenon.  The  law  in  question 
can  be  expressed  by  the  formula, 

M  =  a  —  b  sin^  X; 
w  being  the  angular  velocity  of  a  spot  at  the  latitude  X,  and  a 
and  b  being  constants,  having  the  same  value  (a— S57'.6  and 
/;  =  157'.3)  over  the  wliole  surface  of  the  sun.  These  constants 
may  vary  slowly  witli  the  time,  but  I  have  not  studied  their 
variations. 

"Admitting,  as  we  shall  see  farther  on,  that  the  velocity  of 
a  spot  is  the  same  as  the  mean  velocity  of  that  zone  of  the 
photosphere  in  which  it  is  formed,  we  see : 

"  1.  Tliat  the  contiguous  strips  of  the  photosphere  are  ani- 
mated with  a  velocity  of  rotation  nearl}'  constant  for  each  hla- 
ment,  at  least  during  a  period  of  several  months  or  years,  but 
varying  with  the  latitude  from  one  strip  to  another. 

"  2.  That  these  strips  move  nearly  parallel  to  the  equator, 
and  never  give  indications  of  currents  constantly  directed  tow- 
ards either  pole,  as  in  the  ujoper  regions  of  our  atmosphere. 

"  3.  That  the  spots  are  hollow,  or  at  least  that  the  black  nu- 
cleus is  perceptibly  depressed  in  respect  to  the  photosphere. 

"  Tlie  diminution  in  the  rate  of  superficial  rotation,  more 
and  more  marked  towards  the  poles,  and  the  absence  of  all 
motion  from  the  equator,  can  only  proceed  from  the  vertical 
ascent  of  materials  rising  incessantly  from  a  great  depth  tow- 
ards all  points  of  the  surface.  It  is  sufficient  that  this  depth 
goes  on  increasing  from  the  equator  towards  the  poles,  follow- 
ing a  law  analogous  to  tliat  of  the  rotation,  in  order  that  it 
may  produce  at  the  surface  a  retardation  increasing  with  the 
latitude.  This  retardation  is  about  two  days  in  each  rotation 
at  forty-five  degrees  of  latitude.  The  mass  of  the  sun,  being 
formed  principally  of  metallic  vapors  condensable  at  a  certain 
temperature,  and  that  temperature  being  reached  at  a  certain 
level  in  consequence  of  the  exterior  cooling,  there  ought  to  be 
established  a  double  vertical  movement  of  ascending  vapors, 
which  go  to  form  a  cloud  of  condensed  matter  susceptible  of 


VIEWS  ON  THE  PHYSICAL  CONSTITUTION  OF  THE  SUN.   275 

intense  radiation,  and  of  condensed  products  which  fall  back 
in  the  form  of  rain  into  the  interior.  The  latter  are  stopped 
at  the  depth  at  wliich  they  meet  a  temperature  high  enough 
to  vaporize  them  anew,  and  afterwards  force  them  to  reascend. 
As  almost  the  entire  mass  of  tlie  sun  partakes  of  this  double 
movement,  the  heat  radiated  by  the  cloud  will  be  borrowed 
from  this  mass,  and  not  from  a  superficial  layer,  the  tempera- 
ture of  which  would  rapidly  fall,  and  which  would  soon  con- 
dense into  a  complete  crust.  Hence  the  formation  and  sup- 
port of  the  photosphere,  and  the  constancy  and  long  duration 
of  its  radiation,  which  is  also  partly  fed  by  the  slow  contrac- 
tion of  the  whole  mass  of  the  sun, 

"  The  contiguous  bands  of  the  photosphere  being  animated 
with  different  velocities,  there  results  a  multitude  of  circular 
gyratory  movements  around  a  vertical  axis  extending  to  a 
great  depth,  as  in  our  rivers  and  in  the  great  upper  currents 
of  our  atmosphere.  These  whirlpools,  which  tend  to  equalize 
the  differences  of  velocity  just  spoken  of,  follow  the  currents 
of  the  photosphere  in  the  same  way  that  whirlpools,  and  the 
whirlwinds,  tornadoes,  "ud  cyclones  of  our  atmosphere  follow 
the  upper  currents  in  which  they  originate.  Like  these,  they 
are  descending,  as  I  have  proved  (against  the  meteorologists) 
by  a  special  study  of  these  terrestrial  phenomena.  They  carry 
down  ii:to  the  depths  of  the  solar  mass  the  cooler  materials  of 
the  upper  layers,  formed  principally  of  hydrogen,  and  thus 
produce  in  their  centre  a  decided  extinction  of  light  and  heat 
as  long  as  the  gyratory  movement  continues.  Finally,  the 
hydrogen  set  free  at  the  base  of  the  whirlpool  becomes  re- 
heated at  this  great  depth,  and  rises  up  tumultuously  around 
the  whirlpool,  forming  irregular  jets  which  appear  above  the 
chromosphere.     These  jets  constitute  the  protuberances. 

"The  whirlpools  of  the  sun,  like  those  on  the  earth,  are  of 
all  dimensions,  from  the  scarcely  visible  pores  to  the  enormous 
spots  which  we  see  from  time  to  time.  They  have,  like  those 
of  the  earth,  a  marked  tendency  first  to  increase,  and  then  to 
break  up,  and  thus  form  a  row  of  spots  extending  along  the 
same  parallel.    The  penumbra  is  due  to  a  portion  of  the  photo- 


276  THE  SOLAR  SYSTEM. 

sphere  which  forms  around  tlieir  conical  surface  at  a  lower 
level,  on  account  of  the  lowering  of  the  temperature  produced 
by  the  whirlpool.  Sometimes  in  this  sort  of  luminous  slieath  we 
see  traces  of  the  whirling  movement  going  on  in  the  interior. 

"  It  is  more  difficult  io  account  for  the  periodicity  of  the 
spots.  It  "oems  to  me  that  it  nnist  depend  upon  fluctuations  in 
the  form  of  the  interior  Layer,  to  which  the  condensed  matter 
of  the  photosphere  falls  in  the  form  of  rain.  This  flow  of 
materials  from  above  must  alter,  little  by  little,  the  velocity 
of  rotation  of  this  layer.  If  its  compression  is  changed  in  the 
course  of  time,  and  if  it  becomes  rounder,  the  variations  in 
the  superficial  velocity  of  tlie  photosphere,  as  well  as  the  gyra- 
tory movements,  will  diminisli  in  intensity  and  frequency. 

"A  time  will  at  length  arrive  when  the  vertical  movements 
which  feed  the  photosphere  will  become  more  and  more  hin- 
dered. The  cooling  will  then  be  purely  superficial,  and  the 
surface  of  the  sun  will  harden  into  a  continuous  crust. 

"Paris,  February,  1877." 

Views  of  Professor  Youmj. — "  1.  It  seems  to  me  almost  dem- 
onstrated, as  a  consequence  of  the  low  mean  density  of  the 
sun  and  its  great  force  of  gravity,  that  the  central  portions  of 
that  body,  and,  in  fact,  all  but  a  comparatively  thin  shell  near 
the  surface,  must  be  in  a  gaseous  condition,  and  the  gases  at 
so  high  a  temperature  as  to  remain  for  the  most  part  dissoci- 
ated from  eacli  other,  and  incapable  of  cliemical  interaction. 
Under  tlie  influence  of  tlie  great  pressure  and  high  tempera- 
ture, however,  their  density  and  viscosity  are  probably  such  as 
to  render  their  meclianical  behavior  more  like  that  of  such 
substances  as  tar  or  lioney  than  that  of  air,  as  we  are  famil- 
iar with  it. 

"  2.  Tlie  visible  surface  of  tlie  sun,  the  photosphere,  is  com- 
posed of  clouds  formed  by  the  condensation  and  combination 
of  such  of  the  solar  gases  as  are  cooled  sufficiently  by  their 
radiation  into  space.  These  clouds  are  suspended  in  the  mass 
of  uncondensed  gases  like  the  clouds  in  our  own  atmosphere, 
and  probably  have,  for  the  most  part,  the  form  of  a])proximate- 
ly  vertical  columns,  of  irregular  cross -section,  and  a  length 


VIEWS  OX  THE  PHYSICAL  CONSTITUTION  OF  THE  SUN.  277 

many  times  exceeding  tlicir  diameter.  The  liquid  and  solid 
particles  of  M'liicli  they  are  made  up  descend  continually,  their 
places  being  constantly  supplied  by  fresh  condensation  from 
the  Tis-jcnding  cirrents  whicli  rise  between  the  cloud-columns. 
From  the  under-surface  of  the  photosphere  there  must  be  aiL 
immense  precipitation  of  what  may  be  called  solar  '  rain  and 
snow,'  which  descends  into  the  gaseous  core,  and  by  the  inter- 
nal heat  is  re-evaporated,  decomposed,  and  restored  to  its  origi- 
nal gaseous  condition ;  the  heat  lost  by  the  surface  radiation 
being  replaced  mainly  by  the  mechanical  work  due  to  the 
gradual  diminution  of  the  sun's  bulk,  and  the  thickening  of 
the  photosphere.  I  do  not  know  any  means  of  determining 
the  thickness  of  the  photospheric  shell,  but,  from  the  phenom- 
ena of  the  spots,  judge  that  it  can  hardly  be  less  than  ten 
thousand  miles,  and  that  it  may  be  much  more. 

"  3.  The  weight  of  the  cloud-shell,  and  the  resistance  offered 
to  the  descending  products  of  condensation,  act  to  produce  on 
the  enclosed  gaseous  core  a  constrictting  pressure,  wliicli  forces 
the  gases  upwards  through  the  intervals  between  the  clouds 
with  great  velocity ;  so  that  jets  or  blasts  of  heated  gas  con- 
tinually ascend  all  over  the  sun's  surface,  the  same  material 
subsequently  redescending  in  the  cloud-columns,  partly  con- 
densed into  solid  or  liquid  particles,  and  partly  uncondensed, 
but  greatly  cooled.  It  seems  also  not  unlikely  that  in  the  up- 
per part  of  the  channels  through  which  the  ascending  currents 
rush,  there  may  often  occur  the  mixture  of  different  gases 
cooled  by  expansion  to  temperatures  sufficiently  below  the 
dissociation  point  to  allow  of  their  explosive  combination. 

"4.  The  'chromosphere 'is  simply  the  layer  of  uncondensed 
gases  which  overlies  the  photosphere,  though  separated  from 
it  by  no  definite  surface.  The  lower  portion  of  the  chromo- 
sphere is  rich  in  all  the  vapors  and  gases  which  enter  into  the 
sun's  composition ;  but  at  a  comparatively  small  height  the 
denser  and  less  permanent  gases  disappear,  leaving  in  the  up- 
per regions  only  hydrogen  and  some  other  substances  not  as 
yet  identified.  The  dark  lines  of  the  solar  spectrum  originate 
mainly  in  the  absorption  produced  by  the  denser  gases  which 


278  THE  SOLAB  SYSTEM. 

bathe  the  photospheric  (;louds,  and  these  metallic  vai">ors  are 
only  occasionally  carried  into  the  upper  regions  by  ascending 
jets  of  nnusual  violence.  When  this  occurs,  it  is  almost  in- 
variably in  connection  with  a  solar  spot.  The  prominences 
are  merely  heated  masses  of  the  hydrogen  and  other  chromo- 
spneric  gases,  carried  to  a  considerable  height  by  the  ascend- 
ing currents,  and  apparently  floating  in  the  '  coronal  atmos- 
phere,' which  interpenetrates  and  overtops  the  chromosphere. 

"  5.  I  do  not  know  what  to  make  of  the  corona.  Its  spec- 
trum proves  that  a  considerable  portion  of  its  light  conies 
from  some  exceedingly  rare  form  of  gaseous  matter,  which 
cannot  be  identified  with  anything  known  to  terrestrial  chem- 
istry ;  and  this  gas,  whatever  it  may  be,  exists  at  a  height  of 
not  less  than  a  million  of  miles  above  the  solar  surface,  con- 
stituting the  'coronal  atmosphere.'  Another  portion  of  its 
light  appears  to  be  simply  reflected  sunshine.  But  by  what 
forces  the  peculiar  radiated  structure  of  the  corona  is  deter- 
mined, I  have  no  definite  idea.  The  analogies  of  comets'  tails 
and  auroral  streamers  both  appear  suggestive ;  but,  on  the  other 
hand,  the  spectra  of  the  corona,  the  aurora  borealis,  the  com- 
ets, and  the  nebulai  are  all  difl^erent — no  two  in  the  least  alike. 

"  6.  As  to  sun-spots,  there  can  be  no  longer  any  doubt,  1 
think,  that  they  are  cavities  in  the  upper  surface  of  the  photo- 
sphere, and  that  their  darkness  is  due  simply  to  the  absorbing 
action  of  the  gases  and  vapors  which  fill  them.  It  is  also  cer- 
tain that  very  commonly,  if  not  invariably,  there  is  a  violent 
uprush  of  hydrogen  and  metallic  vapors  all  around  the  outer 
edge  of  the  penumbra,  and  a  considerable  depression  of  the 
chromosphere  over  the  centre  of  the  spot ;  probably,  also,  there 
is  a  descending  current  through  its  centre.  As  to  the  cause 
of  the  spots,  and  the  interpretation  of  their  telescopic  details, 
I  am  unsatisfied.  The  theory  of  Faye  appears  to  me,  on  the 
whole,  the  most  reasonable  of  all  that  have  yet  been  proposed ; 
but  I  cannot  reconcile  it  with  the  want  of  sj^stematic  rotation 
in  the  spots,  or  their  peculiar  forms.  Still,  it  undoubtedly  has 
important  elements  of  truth,  and  may  perhaps  be  modified  so 
as  to  meet  these  difficulties.    As  to  the  periodicity  of  the  spots, 


VIEWS  OX  THE  PHYSICAL  CONSTITUTION  OF  THE  SUN.  279 

I  am  unable  to  think  it  due  in  any  way  to  planetary  action ; 
at  least,  the  evidence  appears  to  nie  wholly  insufficient  as  yet; 
but  I  have  no  hypothesis  to  offer.  Nor  have  I  any  theory  to 
propose  to  account  for  the  certain  connection  between  disturb- 
ances of  the  solar  surface  and  of  terrestrial  magnetism. 

"  7.  As  to  the  temperature  of  the  sun's  surface,  I  have  no 
settled  opinion,  except  tiiat  I  think  it  must  be  much  higher 
than  that  of  the  carbon  points  in  the  electric  light.  The  esti- 
mates of  those  who  base  their  calculations  on  Newton's  law  of 
cooling,  which  is  confessedly  a  mere  approximation,  seem  to 
me  manifestly  wrong  and  exaggerated ;  on  the  other  hand,  the 
very  low  estimates  of  the  French  physicists,  who  base  their 
calculations  on  the  equation  of  Dnlong  and  Petit,  seem  to  me 
hardly  more  trustworthy,  since  their  whole  result  depends 
u])on  the  accuracy  of  a  numerical  exponent  determined  by  ex- 
periment at  low  temperatures  and  under  circumstances  differ- 
ing widely  from  those  of  the  sun's  surface.  The  process  is  an 
unsafe  extrapolation.  The  sensible  constancy  of  the  solar 
radiation  seems  to  be  fairly  accounted  for  on  the  hypothesis 
of  slow  contraction  of  the  sun's  diameter, 

"  8.  I  look  upon  the  accelerated  motion  of  the  sun's  equator 
as  the  most  important  of  the  unexplained  facts  in  solar  phys- 
ics, and  am  persuaded  that  its  satisfactory  elucidation  will  carry 
with  it  the  solution  of  most  of  the  other  problems  still  pending. 

"  Such,  in  brief,  are  my  '  opinions ;'  but  many  of  them  I 
hold  with  little  confidence  and  tenacity,  and  anxiously  await 
more  light,  especially  as  regards  the  theory  of  the  sun's  rota- 
tion, the  cause  and  constitution  of  the  spots,  and  the  nature  of 
the  corona.  Tlie  only  peculiarity  in  my  views  lies,  I  think, 
in  the  importance  I  assign  to  the  effects  of  the  descending 
products  of  condensation,  which  I  conceive  to  form  virtually 
a  sort  of  constricting  skin,  producing  pressure  upon  the  gas- 
eor.s  mass  beneath,  sometliing  as  the  film  of  a  bubble  com- 
presses the  enclosed  air.  To  the  pressure  thus  produced  I 
ascribe  mainly  the  eruptive  phenomena  of  the  chromosphere 
and  prominences. 

"  Df.rtmouth  College,  March,  1877." 


280  THE  SOLAR  SYSTEM. 

Vieios  of  Professor  Lan'jhnj. — "It  seems  to  me  that  we  have 
now  evidence  on  which  to  pass  final  adverse  judgment  on 
views  wliich  regard  the  pliotosphere  as  an  incandescent  li(piid, 
or  the  spots  as  analogous  cither  to  scoriae  mutter,  on  tlie  one 
hand,  or  to  clouds  above  the  luminous  surface,  on  tlie  other. 
According  to  direct  telescopic  evidence,  the  pliotospliere  is 
purely  vaporous,  and  I  consider  these  upper  vapors  to  be 
ligliter  than  the  thinnest  cirri  of  our  own  sky.  The  obser- 
vation of  faculiu  allies  them  avid  the  whole  'granular'  cloud 
structure  of  the  surface  most  intimately  with  chromospheric 
forms,  seen  by  the  spectroscope,  and  associates  both  with  the 
idea  of  an  everywhere-acting  system  of  currents  which  trans- 
mit the  internal  heat,  generated  by  condensation,  to  the  sur- 
face, and  take  back  the  cold,  absorbent  matter.  This  vertical 
circulation  goes  to  a  depth,  I  think,  sensible  even  by  compari- 
son with  the  solar  diameter.  It  coexists  with  approximately 
horizontal  movements  obsei'ved  in  what  may  be  called  the 
successive  upper  photospheric  strata  in  the  vicinity  of  spots. 
The  spots  give  evidence  of  cyclonic  action  such  as  could  only 
occur  in  a  lluid.  Their  darkness  is  due  to  the  presence,  in 
unusual  depth,  of  the  same  obscuring  atmosphere  which  forms 
the  gray  medium  in  which  the  luminous  photospheric  forms 
seem  suspended,  and  which  we  here  look  through,  where  it 
tills  openings  in  the  photospheric  stratum,  down  to  regions 
of  the  solar  interior  made  visible  by  the  dim  light  of  clouds 
of  hnninous  va])or,  pre(;ipitated  in  lower  strata  where  the  dew- 
point  has  been  altered  by  changed  conditions  of  temperature 
and  pressure.  All  observation  and  all  legitimate  inference 
go  to  show  that  the  sun  is  gaseous  throughout  its  mass,  though 
by  this  it  is  not  meant  to  deny  the  probable  precipitation  of 
cooling  photos])heric  vapors  in  something  analogous  to  rain ; 
a  condition  perhaps  necessary  to  the  maintenance  of  the  equi- 
librium of  the  interchange  of  cold  and  lieated  matter  between 
exterior  and  interior ;  nor  is  it  meant  that  the  conditions  of  a 
perfect  fluid  are  to  be  expected,  where  these  are  essentially 
modified  (if  by  no  other  cause)  by  the  viscosity  due  to  extreme 
heat.     The  temperature  of  the  sun  is,  in  my  view,  necessarily 


riEUS  ox  THE  PHYSICAL  CONSTITUTION  OF  THE  SUN.   281 

much  greater  than  that  assigned  hy  the  numerous  physicists, 
wlio  maintain  it  to  he  comparahle  with  tliat  ohtainahlo  in  the 
laboratory  furnace;  but  we  cannot  confideutly  assign  any  \ip- 
]icr  hniit  to  it  until  i)hysics  has  advanced  beyond  its  present 
merely  empirical  rides  connecting  ennssion  and  temperature; 
for  this,  and  not  the  lack  of  accurate  data  from  physical 
astronomy,  is  the  r^ourcc  of  nearly  all  the  ol)scurity  now  at- 


Fio.  72.— Solar  spot,  aftci-  Langley. 

tending  this  important  question.  No  theory  of  the  solar  con- 
stitution which  is  free  from  some  objecti(»'>  has  yet  been  pro- 
posed ;  but  if  the  master-key  to  the  diverse  proble;ns  it  pre- 
sents has  not  been  found,  it  is  still  true,  I  think,  that  the  one 
which  unlocks  most  is  that  of  M.  Favc. 

'"  Of  the  potential  energy  of  the  sun,  we  may  say  that  we 
believe  it  to  be  sufficient  for  a  supply  of  the  present  heat  dur- 
ing periods  to  be  counted  by  millions  of  years.     J3ut  what  im- 


282  THE  SOLAR  SYSTEM. 

mediately  concerns  us  is  the  constancy  of  the  rate  of  conver- 
sion of  tliis  potential  into  actual  i-adiant  energy,  as  we  receive 
it,  for  on  this  depends  the  uniformity  of  the  conditions  under 
which  we  exist.  Now,  this  uniformity  in  turn  depends  on 
the  equality  of  the  above-mentioned  interchanges  between  the 
solar  surface  and  the  interior,  an  equality  of  whose  constancy 
we  know  nothing  save  by  limited  experience.  The  most  im- 
portant statement  with  reference  to  the  sun,  perhaps,  which 
we  can  make  with  certainty  is  even  a  negative  one.  It  is 
that  we  have  no  other  than  empirical  grounds,  in  the  present 
state  of  knowledge,  for  believing  in  the  uniformity  of  the 
solar  radiation  in  prehistoric  periods  and  in  the  future. 

"  The  above  remarks,  limited  as  they  are,  appear  to  me  to 
cover  nearly  all  the  points  as  to  the  sun's  physical  constitu- 
tion (outside  of  the  positive  testimony  of  the  spectroscope)  on 
which  we  are  entitled  to  speak  with  confidence,  even  at  the 
present  time." 


THE  I'LANET  MEliCUIiY. 


283 


CHAPTER  III. 


THE  INNER  GROUP  OF  PLANETS. 

§  1.  T/ic  Planet  Mercury. 

Mercury  is  the  nearest  known  planet  to  the  snn,  and  the 
smallest  of  the  eight  large  planets.  Its  mean  distance  from 
the  sun  is  40  millions  of 
miles,  and  its  diameter  about 
one-third  that  of  the  earth. 
It  was  well  known  to  the  an- 
cients, being  visible  to  the 
naked  eye  at  favorable  times, 
if  the  observer  is  not  in  too 
high  a  latitude.  The  central 
and  northern  regions  of  Eu- 
rope are  so  unfavorably  sit- 
uated for  seeing  it  that  it  is 
said  Copernicus  died  without 
ever  having  been  able  to  ob- 
tain a  view  of  it.  The  difh- 
culty  of  seeing  it  arises  from 
its  proximity  to  the  sun,  as  it  seldom  sets  more  than  an  hour 
and  a  half  after  the  sun,  or  rises  more  than  that  length  of 
time  before  it.  Hence,  when  the  evening  is  sufficiently  ad- 
vanced to  allow  it  to  be  seen,  it  is  commonly  so  near  the  hori- 
zon as  to  be  lost  in  the  vapors  which  are  seen  in  that  direction. 
Still,  by  watching  for  favorable  moments,  it  can  be  seen  sev- 
eral times  in  the  course  of  the  year  in  any  part  of  the  United 
States, 
sunset : 


Fia.  73.— Orbits  of  the  four  inner  planets,  il- 
lustniting  the  eccentricity  of  those  of  Mercu- 
ry    ul  Mars. 


The  following  are  favorable  times  for  seeing  it  after 


1877 May  3d,  August  2Gtli,  December  SSth. 

1878 April  14tli,  August  9th,  December  9th. 

1879 March  28th,  July  23d,  November  21st. 


284  THE  SOLAR  SYSTEM. 

The  corresponding  times  in  subsequent  years  may  be  found 
by  subtracting  18  days  from  tlie  dates  for  each  year;  that  is, 
they  will  occur  18  days  earlier  in  1879  than  in  1878;  18  days 
earlier  in  1880  than  in  1879,  and  so  on.  It  is  not  necessary 
to  1  jok  on  the  exact  days  we  have  given,  as  the  planet  is  gen- 
erally visible  for  fifteen  or  twenty  days  at  a  time.  Each  date 
given  is  about  the  middle  of  tlie  period  of  visibility,  which  ex- 
tends a  week  or  ten  day,:  on  each  side.  The  best  time  for  look- 
ing is  in  the  evening  twilight,  about  three-quarters  of  an  hour 
after  sunset,  the  spring  is  in  this  respect  much  more  favorable 
than  autumn. 

Aspect  of  Mercury. — IVIercury  shines  with  a  brilliant  white 
light,  brighter  than  that  of  any  fixed  star,  except,  perhaps, 
Sirius.  It  does  not  seem  so  bright  as  Sirius,  because  it  can 
never  be  seen  at  night  except  very  near  the  horizon.  Owing 
to  the  great  eccentricity  of  its  orbit  and  the  great  variations  of 
its  distance  from  the  earth,  its  brilliancy  varies  considerably ; 
but  the  favorable  times  we  have  indicated  arc  near  those  of 
greatest  brightness. 

Viewed  with  a  telescope  under  favorable  conditions,  Mer- 
cury is  seen  to  have  phases  like  the  moon.  When  beyond  the 
sun,  it  seems  round  and  small,  being  only  about  5"  in  diame- 
ter. When  seen  to  one  side  of  the  sun,  near  its  greatest  fip- 
parent  angular  distance,  it  appears  like  a  half-moon.  When 
nearly  between  the  sun  and  earth,  its  diameter  is  between  10'' 
and  12",  but  only  a  thin  crescent  is  visible.  The  manner  in 
which  tliese  various  phases  are  connected  with  the  position  of 
the  planet  relative  to  the  earth  and  sun  is  the  same  as  in  the 
case  of  Venus,  and  will  be  shown  in  the  next  section. 

Rotation,  Figure,  Atmosphere,  etc. — About  the  beginning  of 
the  present  century  Schroter,  the  celebrated  astronomer  of 
Lilienthal,  who  made  the  telescopic  study  of  the  planets  a 
speciality,  thought  that  at  times,  when  Mercury  presented  the 
aspect  of  a  crescent,  the  south  horn  of  this  crescent  seemed 
blunted  at  certain  ijitervals.  He  attributed  this  appearance  to 
the  shadow  of  a  lofty  mountain,  and  by  observing  the  times 
of  its  return  was  led  to  the  conclusion  that  the  planet  revolved 


TRANSITS  OF  MERCURY.  285 

on  its  axis  in  24:  hours  5  minutes.  He  also  estimated  the 
height  of  the  mountain  at  twelve  miles.  But  the  more  power- 
ful instruments  of  modern  times  have  not  confirmed  these 
conclusions,  and  they  are  now  considered  as  quite  doubtful,  if 
not  entirely  void  of  foundation.  That  is,  we  must  regard  the 
time  of  rotation  of  Mercury  on  its  axis,  and,  of  course,  the 
position  of  that  axis,  as  not  known  with  certainty,  but  as  per- 
haps very  nearly  24  liours. 

Tlie  supposed  atmosphere  of  Mercury,  the  deviation  of  its 
body  from  a  spherical  form,  and  many  other  phenomena 
which  observers  have  described,  must  be  received  with  the 
same  scepticism.  No  deviation  from  a  spherical  form  can  be 
considered  as  proved,  the  discordance  of  the  measures  showing 
that  the  supposed  deviations  arc  really  due  to  errors  of  obser- 
vation. So,  also,  the  appearances  which  numy  observers  have 
attributed  to  an  atmosphere  are  all  to  be  regarded  as  optical 
illusions,  or  as  due  to  the  imperfections  of  the  telescope  made 
use  of.  From  measures  of  its  light  it  various  phases  Zollner 
has  been  led  to  the  conclusion  that  Mercury,  like  our  moon, 
is  devoid  of  any  atmosphere  sufficiently  dense  to  reflect  the 
light  of  the  sun.  If  this  doubt  and  uncertainty  seems  surpris- 
ing, if  must  be  remembered  that  the  nearness  of  this  planet  to 
the  sun  rendc/s  it  a  very  difficult  object  to  observe  with  accu- 
racy. We  must  look  at  it  either  in  the  daytime,  when  the  air 
is  disturbed  by  the  sun's  rays,  or  in  the  early  evening,  when  the 
planet  is  very  near  the  horizon,  and  therefore  in  an  unfavorable 
situation. 

Transits  of  Mercunj. — Transits  of  this  planet  across  the  face  , 
of  the  sun  are  much  more  frequent  than  those  of  Venus,  the 
average  interval  between  successive  transits  being  less  than  ten 
years,  and  the  lo!igest  interval  thirteen  years.  These  transits 
are  always  looked  upon  with  great  interest  by  astronomers,  on 
account  of  the  questions  to  which  they  have  given  rise.  From 
the  earliest  ages  in  which  it  was  known  that  Mercury  moved 
around  the  sun,  it  was  evident  that  it  must  sometimes  pass  be- 
tween the  earth  and  the  sun ;  but  its  diameter  is  too  small  to 
admit  of  its  being  seen  in  this  position  with  the  naked  eye. 


286 


THE  SOLAR  SYSTEM. 


The  first  actual  observation  of  Mercury  projected  on  the  face 
of  the  sun  was  made  by  Gassendi,  on  November  7tli,  1631. 
Tlis  mode  of  observation  was  that  already  described  for  viewing 
the  solar  spots,  the  image  of  the  sun  being  thrown  on  a  screen 
by  means  of  a  small  telescope.  lie  came  near  missing  liis  ob- 
servation, owing  to  his  having  expected  that  the  planet  would 
look  much  larger  than  it  did.  The  imperfect  telescopes  of 
that  time  surrounded  every  brilliant  object  with  a  band  of 
diffused  light  Avhicli  greatly  increased  its  apparent  magni- 
tude, so  that  Gassendi  had  no  idea  how  small  the  planet  really 
was. 

Gassendi's  observation  was  hardly  accurate  enough  to  be  of 
any  scientific  value  at  the  present  time.  It  was  not  till  1677 
that  a  really  good  observation  was  made.  Halley,  of  England, 
in  that  year  was  on  the  island  of  St.  Helena,  and,  being  pro- 
vided with  superior  instruments,  was  fortunate  enough  to  make 
a  complete  observation  of  a  transit  of  Mercury  over  the  sun 
which  occurred  on  November  7th.  We  have  already  men- 
tioned the  great  accuracy  which  lie  attributed  to  his  observa- 
tion, and  the  phenomenon  of  the  black  drop  which  he  was  the 
first  to  see. 

The  following  are  the  dates  at  which  it  has  been  calculated 
that  transits  of  Mercury  will  occur  during  the  remainder  of 
the  present  century.  The  first  transit  will  be  visible  over  the 
whole  United  States,  and  the  second  on  the  Pacific  coast. 


1878 May  Gth. 

1881 November  7th, 

1891 May  i)th. 


1894 November  10th. 

1901 November  4th. 


§  2.  llie  /Supposed  Intra- Mercurial  Planets. 

At  the  present  time  the  greatest  interest  which  attaches  to 
transits  of  Mercurv  arises  from  the  conclusion  which  Lever- 
rier  has  drawn  from  a  profound  comparison  of  transits  ob- 
served before  1848  with  the  motion  of  Mercury  as  determined 
from  the  theory  of  gravitation.  This  comparison  indicates, 
according  to  Leverrier,  that  the  perihelion  of  Mercury  moves 
more  rapidly  by  40"  a  century  than  it  ouglit  to  from  the  grav- 


THE  SUPPOSED  INTRA-MEliCURIAL  PLANETS.         287 

itation  of  all  tlie  known  planets  of  the  system.  He  accounted 
for  this  motion  hy  supposing  a  group  of  small  planets  between 
Mercury  and  the  sun,  and  the  question  whether  such  planets 
exist,  therefore,  becomes  important. 

Apparent  support  to  Leverrier's  theory  is  given  by  the  fact 
that  various  observers  have  within  the  past  century  recorded 
the  passage  over  the  disk  of  the  sun  of  dark  bodies  which  had 
the  appearance  of  planets,  and  which" went  over  too  rapidly  or 
disappeared  too  suddenly  to  be  spots.  But  when  we  examine 
these  observations,  we  iind  that  they  arc  not  entitled  to  the 
slightest  conlidence.  There  is  a  large  class  of  recorded  as- 
tronomical phenomena  which  are  seen  only  by  unskilful  ob- 
servers, with  imperfect  instruments,  or  under  unfavorable  cir- 
cumstances. The  fact  tiiat  they  are  not  seen  by  ])ractised  ob- 
servers with  good  instruments  is  sufficient  proof  that  there  is 
something  wrong  about  them.  NoWj  the  observations  of  in- 
tra-Mercurial  planets  belong  to  this  class.  Wolf  has  collected 
nineteen  observations  of  unusual  appearances  on  the  sun,  ex- 
tending from  1761  to  18G5,  but,  with  two  or  three  exceptions, 
the  observers  are  almost  unknown  as  astronomers.  In  at  least 
one  of  these  cases  the  observer  did  not  ])rofcss  to  have  seen 
anything  like  a  planet,  but  only  a  cloud-like  appearance.  On 
the  other  hand,  for  lifty  years  past  the  sun  has  been  constant- 
ly and  assiduously  observed  by  such  men  as  Schwabe,  Carring- 
ton,  Secclii,  and  Spoerer,  none  of  whom  have  ever  recorded 
anything  of  the  sort.  That  planets  in  such  numbers  should 
pass  over  the  solar  disk,  and  be  seen  by  amateur  observers, 
and  yet  escape  all  these  skilled  astronomers,  is  beyond  all 
moral  probability. 

In  estimating  this  probability  we  must  remember  that  a 
real  planet  appearing  on  the  sun  would  be  far  more  likely  to 
be  recognized  by  a  practised  than  by  an  unpractised  observer, 
much  as  a  new  species  of  plant  or  animal  is  more  likely  to  be 
recognized  by  a  naturalist  than  by  one  who  is  not  such.  One 
not  accustomed  to  the  close  study  of  the  solar  spots  might 
have  some  difficult}'  in  distinguish  g  an  unusually  round  spot 
from  a  planet.    He  is  also  liable  to  be  deceived  in  various 


288  THE  SOLAR  SYSTEM. 

ways,*  For  instance,  the  sim,  by  his  apparent  diurnal  motion, 
presents  different  parts  of  tlie  edge  of  his  disk  to  the  hori- 
zon in  the  course  of  a  day ;  lie  seems,  in  fact,  in  the  north- 
ern hemisphere  to  tm*n  round  in  the  same  direction  with  the 
hands  of  a  watch.  Hence,  if  a  spot  is  seen  near  the  edge  of 
his  disk  it  will  seem  to  be  in  motion,  though  really  at  rest. 
On  the  other  hand,  should  an  experienced  observer  see  a  planet 
projected  on  the  sun's  face,  he  could  hardly  fail  to  recognize  it 
in  a  moment ;  and  should  any  possible  doubt  exist,  it  would  be 
removed  by  a  very  brief  scrutiny. 

The  strongest  argument  against  these  appearances  bcitig 
planets  is,  that  the  transit  of  a  planet  in  si  h  a  position  could 
not  be  a  rare  phenomenon,  but  would  necessarily  repeat  itself 
at  certain  intervals,  depending  on  its  distance  from  the  sun 
and  the  inclination  of  its  orbit.  For  instance,  supposing  an 
inclination  of  10°,  which  is  greater  than  that  of  any  of  the 
principal  planets,  and  a  distance  from  the  sun  one-half  that 
of  Mercury,  the  planet  would  pass  over  the  face  of  the  sun, 
on  the  average,  about  once  a  year,  and  its  successive  transits 
would  '  ccur  either  very  near  the  same  day  of  the  year,  or  on 
a  certain  day  of  the  opposite  season.  The  supposed  transits 
to  which  we  have  referred  occur  at  all  seasons,  and  if  we  sup- 
pose them  real,  we  must  suppose,  as  a  logical  consequence, 
that  the  transits  of  these  several  planets  are  repeated  many 
times  a  year,  and  yet  constantly  elude  the  scrutiny  of  all  good 
observers,  though  occasionally  seen  by  unskilled  ones.  This  is 
a  sufficient  reduclio  ad  absurdum  of  the  theoiy  of  their  reality. 

It  is  very  certain,  then,  that  if  the  motion  of  the  perihelion 
of  Mercury  is  due  to  a  group  of  planets,  they  are  each  so 
small  as  to  be  invisible  in  transits  across  the  sun.     They  nmst 

*  Some  readers  may  recall  Butler's  sarcastic  poem  of  the  "Elephant  in  the 
Moon,"  as  illusti-ative  of  the  jmssibility  of  an  observer  being  deceived  by  some  pe- 
culiarity of  his  telescope.  In  one  instance,  about  thirty  years  since,  a  telescopic 
observation  of  something  which  we  now  know  must  liave  been  flights  of  distant 
birds  over  the  disk  of  the  sun  was  recorded,  and  jjublished  in  one  of  the  leading 
astronomical  journals,  as  a  wonderful  transit  of  meteors.  The  publication  was 
probably  not  seriously  intended,  the  description  being  a  close  parallel  to  that  of 
the  satirical  poet.     See  Astronomische  Nachrichten,  No.  549. 


THE  PLANET  VENUS.  289 

also  be  so  small  as  to  be  invisible  during  total  eclipses  of  the 
sun,  because  they  have  always  failed  to  sh'^nv  themselves  then. 
But  to  produce  the  observed  effect  on  Mercury,  their  total 
mass  must  be  three  or  four  times  that  of  Mercury.     Being  so 
small  individually,  and  so  large  in  the  aggregate,  their  num- 
ber must  be  counted  by  thousands ;  and  if  seen  at  all,  they 
will  be  seen  only  as  a  cloud-like  mass.     Now,  in  the  zodiacal 
light  we  have  such  a  mass,  and  the  question  arises  whether  the 
matter  which  reflects  this  light  can  be  that  which  affects  the 
'motions  of  Mercury.     Although  the  affirmative  of  this  ques- 
tion  involves  nothing  intrinsically  improb'^blc,  it  cannot  be 
accepted  without  further  investigation.      The  delicate  point 
involved  is,  that  unless  we  suppose  the  hypothetical  group  of 
planetoids  to  move  nearly  in  the  plane  of  the  orbit  of  Mercury, 
they  must  change  the  node  of  that  planet  as  well  as  its  peri- 
helion.    Now,  the  observations  discussed  by  Leverrier  do  not 
show  any  motion  of  the  node  above  that  due  to  the  action  of 
the  known  planets.     We  thus  reach  the  enforced  conclusion 
that  if  the  motion  of  the  perihelion  is  due  to  the  cause  as- 
signed by  Leverrier,  the  planetoids  which  cause  it  must,  in  the 
mean,  move  in  nearly  the  same  plane  with  Mercury.     But  it 
has  not  yet  been  shown  that  the  axis  of  the  zodiacal  light  de- 
viates from  the  ecliptic  by  so  great  an  angle  as  the  orbit  of 
Mercury,  namely  7°.    A  great  deal  of  research — more,  in  fact, 
than  is  likely  to  be  applied  to  the  subject  during  the  present 
generation — will  be  required  befoi'e  the  question  can  be  settled. 

§  3.  The  Planet  Venus. 

The  planet  Yenus  moves  around  the  sun  about  half-way 
between  the  orbits  of  Mercury  and  the  earth,  its  mean  distance 
from  the  sun  being  67  millions  of  miles.  Its  orbit  is  more 
nearly  circular  than  that  of  any  of  the  other  principal  planets. 
It  is  very  nearly  the  size  of  the  earth,  its  diameter  being  little, 
if  any,  more  than  four  per  cent,  less  than  that  of  our  globe. 
Next  to  the  sun  and  moon,  it  is  the  most  brilliant  object  in 
the  heavens,  sometimes  casting  a  very  distinct  shadow.  It 
never  recedes  more  than  about  45°  from  the  sun,  and  is,there- 

20 


290  THE  SOLAR  SYSTEM. 

fore,  seen  by  night  only  in  the  western  sky  in  the  evening,  or 
the  eastern  sky  in  the  morning,  according  as  it  is  east  or  west 
of  the  sun.  There  is,  therefore,  seldom  any  difficulty  in  rec- 
ognizing it.  When  at  its  greatest  brilliancy,  it  can  be  clearly 
seen  by  the  naked  eye  In  tlie  daytime,  provided  that  one  knows 
exactly  where  to  look  for  it.  It  was  known  to  the  ancients  by 
the  names  of  Hesperus  and  Phosj^horus,  or  the  evening  and 
the  morning  star,  the  former  name  being  given  when  the 
planet,  being  east  of  the  sun,  was  seen  in  the  evening  after 
sunset,  and  the  latter  when,  being  to  the  west  of  the  sun,  it 
was  seen  in  the  east  before  suiu'ise.  It  is  said  that  before  the 
birth  of  exact  astronomy  IlesjKrus  and  Pliospliorus  were  sup- 
pose'^' to  be  two  different  bodies,  and  that  it  was  not  until 
their  motions  were  studied,  and  the  one  was  seen  to  emerge 
from  the  sun's  rays  soon  after  the  other  was  lost  in  them,  that 
their  identity  was  established. 

As2-)ect  of  Venus. — To  the  unaiued  eye  Venus  presents  the 
appearance  of  a  mere  star,  distinguishable  from  other  stars 
only  by  its  intense  brilliancy.  But  when  Galileo  examined 
this  planet  wMth  his  telescope,  he  found  it  to  exhibit  phases 
like  those  of  the  moon.  Desiring  to  take  time  to  assure  him- 
self of  the  reality  of  his  discovery,  without  danger  of  losing 
his  claim  to  priority  through  some  one  else  in  the  mean  time 
making  it  independently,  he  published  the  following  anagram, 
in  which  it  was  concealed: 

"  HiTcc  iinmntiira  a  me  jam  frustra  leguntur  o.  y." 
(These  unripe  things  are  now  vainly  gathered  by  rae). 

By  transposing  the  letters  of  this  sentence  he  afterwards 
showed  that  they  could  be  made  into  the  sentence, 

"Cynthia;  figurns  asmulatur  mater  amorum" 
(Tiie  mother  of  the  loves  imitates  the  phases  of  Cynthia). 

That  the  disk  of  Yenus  was  not  round  was  first  noticed  by 
Galileo  in  September,  1610.  A  computation  of  its  position 
at  that  time  shows  that  it  must  have  been  a  little  gibbous, 
more  than  half  of  its  face  being  illuminated;    but  after  a 


THE  PLANET  VENUS.  291 

few  months  it  changed  into  a  crescent.  Therefore  Galileo 
could  not  liave  found  it  necessary  to  wait  long  before  explain- 
ing his  anagram. 

Tlie  variations  of  the  aspect  and  apparent  magnitude  of 
Venus  are  very  great.  When  beyond  the  sun,  it  is  at  a  dis- 
tance of  160  millions  of  miles,  and  presents  the  appearance 
of  a  small  round  disk  10"  in  diameter.  When  nearest  the 
earth,  it  is  only  25  millions  of  miles  distant ;  and  if  its  whole 
face  were  visible,  it  would  be  more  than  60"  in  diameter. 


€ 

#  •   # 

Fio.  74.— Phases  of  Venus,  showing;  apparent  flijure  and  majjnitnde  of  the  bright  and  dark 
portioua  of  the  planet  iu  various  points  of  its  orbit. 

But,  being  tlien  on  the  same  side  of  the  sun  with  us,  its  dark 
hemisphere  is  turned  towards  us,  except,  perhaps,  an  extreme- 
ly tliin  crescent  of  the  illuminated  hemisphere.  Between 
these  two  positions  it  goes  through  all  the  intermediate 
phases,  the  universal  rule  of  which  is  that  the  nearer  it  is 
to  the  earth,  the  smallpv  the  proportion  of  its  ai)parent  disk 
which  is  illuminated  ;  but  the  larger  that  disk  would  appear 
could  the  whole  of  it  be  seen.  Its  greatest  brilliancy  occurs 
between  the  time  of  its  greatest  elongation  from  the  sun  and 
its  inferior  conjunction. 

Supiwscd  Relation  of  Venus. — The  earlier  telescopists  natu- 
rally scrutinized  the  planets  very  carefully,  with  a  view  of  find- 
ing whether  there  were  any  inequalities  or  markings  on  their 
surfaces  from  which  the  time  of  rotation  on  their  axes  could 
be  determined.  In  April,  1667,  Cassini  saw,  or  thought  he 
saw,  a  bright  spot  on  Yenus,  by  tracing  which  for  several  suc- 
cessive evenings  he  found  that  the  planet  revolved  in  betAveen 
23  and  24  hours.     Sixty  years  later  Blanchini,  an  Italian  as- 


292  THE  SOLAR  SYSTEM. 

tronomer,  whose  telescope  is  shown  on  page  112,  supposed  that 
he  found  seven  spots  on  the  planet,  which  he  considered  to  be 
seas.  By  watching  them  from  night  to  night,  he  concluded 
that  it  required  more  than  24  days  for  Venus  to  revolve  on 
its  axis.  This  extraordinary  result  was  criticised  by  the  sec- 
ond Cassini,  who  showed  that  Blanchini,  only  seeing  the  plan- 
et a  short  time  each  evening,  and  finding  the  spots  night  after 
night  in  nearly  the  same  position,  concluded  that  it  had  moved 
very  little  from  night  to  night ;  whereas,  in  fact,  it  had  made 
a  complete  revolution,  and  a  little  more.  At  the  end  of  24 
days  it  would  be  seen  in  its  original  position,  but  would  have 
made  25  revolutions  in  the  mean  time,  instead  of  one  only,  as 
Blanchini  supposed.  This  would  make  the  time  of  rotation 
23  hours  2|-  minutes,  while  Cassini  found  23  hours  15  minutes 
from  his  father's  observations. 

Between  1788  and  1793  Schroter  applied  to  Venus  a  mode 
of  observation  similar  to  that  he  used  to  find  the  rotation  of 
Mercury.  Watching  the  sharp  horns  v/hen  the  planet  appear- 
ed as  a  crescent,  he  thought  that  one  of  them  was  blunted  at 
certain  intervals.  Attributing  this  appearance  to  a  high  moun- 
tain, as  in  the  case  of  Mercury,  he  found  a  time  of  rotation 
of  23  hours  21  minutes. 

On  the  other  hand,  Herschel  was  never  able  to  see  any  per- 
manent markings  on  Venus.  He  thought  he  saw  occasional 
spots,  but  tiiey  vai'ied  so  much  and  disappeared  so  rapidly  that 
he  could  not  gather  any  evidence  of  the  rotation  of  the  plan- 
et. He  therefore  supposed  that  Venus  was  surrounded  by  an 
atmosphere,  and  that  whatever  markings  might  be  occasional- 
ly seen  were  due  to  clouds  or  other  varying  atmospheric  phe- 
nomena. 

In  1842,  De  Vico,  of  Rome,  came  to  the  rescue  of  tiie  older 
astronomers  by  publishing  a  series  of  observations  tending  to 
show  that  he  had  rediscovered  the  markijigs  found  by  Blan- 
chini more  than  a  century  before.  lie  deduced  for  the  time 
of  rotation  of  the  planet  23  hours  21  minutes  22  seconds. 

The  best-informed  astronomers  of  the  present  day  look  with 
suspicion  on  nearly  all  these  observations,  being  disposed  to 


THE  PLANET  VENUS.  293 

sustain  the  view  of  Ilerselicl,  though  on  grounds  entirely  dif- 
ferent from  those  on  which  he  founded  it.  It  is  certain  tiiat 
there  are  plenty  of  observers  of  the  present  day,  with  instru- 
ments much  better  than  those  of  their  predecessors,  who  have 
never  been  able  to  see  any  permanent  spots.  The  close  agree- 
ment between  the  times  of  rotation  found  by  the  older  ob- 
servers is  indeed  striking,  and  might  seem  to  render  it  certain 
that  they  must  have  seen  spots  which  lasted  several  days.  It 
must  also  be  admitted  in  favor  of  these  observers  that  a  fine 
steady  atmosphere  is  as  necessary  for  such  observations  as  a 

le  telescope,  and  it  is  possible  that  in  this  respect  the  Italian 
astronomers  may  be  better  situated  than  those  farther  north. 
But  the  circumstance  that  the  deduced  times  of  rotation  in 
the  cases  both  of  Mercury  and  Venus  differ  so  little  from  that 
of  the  earth  is  somewhat  suspicious,  because  if  the  appearance 
were  due  to  any  optical  illusion,  or  imperfection  of  the  tele- 
scope, it  might  repeat  itself  sevei-al  days  in  succession,  and 
thus  give  rise  to  the  belief  that  the  time  of  rotation  was  near- 
ly one  day.  The  case  is  one  on  which  it  is  not  at  present  pos- 
sible to  pronounce  an  authoritative  decision ;  but  the  balance 
of  probabilities  is  largely  in  favor  of  the  view  that  the  rota- 
tation  of  Venus  on  its  axis  has  never  been  seen  or  determined 
by  any  of  the  astronomers  who  have  made  this  planet  an  ob- 
ject of  stud3^* 

Atmosphere  of  Venus. — The  ai)peai*ance  of  Venus  when  near- 
ly between  us  and  the  sun  affords  very  strong  evidence  of  the 
existence  of  an  atmosphere.  The  limb  of  the  planet  farthest 
from  the  sun  is  then  seen  to  be  illuminated,  so  that  it  appears 
as  a  complete  circle  of  light.  If  only  half  the  globe  of  the 
planet  were  illuminated  by  the  sun,  this  appearance  could 
never  present  itself,  as  it  is  impossible  for  an  observer  to  see 
more  than  half  of  a  large  sphere  at  one  view.     There  is  no 

*  The  latest  physical  observations  on  Venns  with  which  I  am  acquainted  are 
those  of  Dr.  Vogel  at  Bothkamp,  in  Part  II.  of  the  "  Botiikamp  Observations" 
(Leipzig,  Engelmann,  1873).  Tiie  result  to  which  these  observations  point  is  that 
the  atmosphere  of  Venus  is  filled  with  clouds  so  dense  that  the  solid  body  of  the 
planet  can  not  be  seen,  and  no  time  of  rotation  can  be  determined. 


294  THE  SOLAR  SYSTEM. 

known  way  in  wliicli  the  snn  can  illuminate  so  nuicli  more 
than  the  half  of  Venus  as  to  permit  a  complete  circle  of  light 
to  1)0  seen  except  by  the  refraction  of  an  atmosphere. 

The  appearance  to  whi*  a  we  allude  was  first  noticed  by 
David  Ilittenhouse,  of  Philadelphia,  while  observing  the  tran- 
sit of  Venus  on  June  3d,  17C9.  When  Venn?  had  entered 
about  half-way  upon  the  sun's  disk,  so  as  to  cut  out  a  notch  of 
the  form  of  a  half-circle,  that  part  of  the  edge  of  the  planet 
which  was  off  the  disk  appeared  illuminated  so  that  the  out- 
line of  the  entire  planet  could  be  seen.  As  this  appearance 
was  not  confirmed  by  other  observers,  it  seems  to  have  excit- 
ed no  attention.  But  it  was  found  by  Miidler  in  1849  that 
when  Venus  was  near  inferior  conjunction,  the  visible  crescent 
extended  through  more  than  a  half-circle.  This  showed  that 
more  than  half  the  globe  of  Venus  was  illuminated  by  the 
sun,  and  Miidler,  computing  the  refractive  power  of  the  atmos- 
phere which  would  be  necessarj'  to  produce  this  effect,  found 
that  it  would  exceed  that  of  our  own  atmosphere ;  the  hori- 
zontal refraction  being  44',  whereas  on  the  earth  it  is  only 
34'.  He  therefore  concluded  that  Venus  was  surrounded  by 
an  atmosphere  a  little  more  dense  than  that  of  the  earth. 

Tiie  next  important  observation  of  the  kind  was  made  by 
Professor  C.  S.  Lyman,  of  Yale  College.  In  December,  18G6, 
Venus  was  very  near  her  node  at  inferior  conjunction,  and 
passed  unusually  near  the  line  drawn  from  the  earth  to  the 
sun.  Examining  the  minute  crescent  of  the  planet  with  a 
moderate-sized  telescope,  he  found  that  he  could  see  the  entire 
(!ircle  of  the  planet's  disk,  an  exceedingly  thin  thread  of  light 
being  stretched  round  the  side  farthest  from  the  sun.  So  far 
as  known,  this  was  the  first  time  that  the  whole  circle  of  Venus 
had  been  seen  in  this  way  since  the  time  of  Rittenhouse.  It 
is  remarkable  that  both  observations  should  have  been  made 
by  isolated  observers  in  America. 

Notwithstanding  the  concurrent  testimony  of  Rittenhouse, 
Miidler,  and  Lyman,  the  bearing  of  their  observations  on  what 
was  to  be  expected  during  the  transit  of  Venus  in  December, 
1874,  was  entirely  overlooked.     Accordingly,  many  of  the  ob- 


THE  PLANET  VENUS.  295 

servers  were  quite  taken  by  surprise  to  find  tliat  wlien  Venus 
M'as  partly  on  and  partly  oft"  the  sun,  the  outline  of  that  part 
of  her  disk  outside  the  sun  could  be  distinguished  by  a  deli- 
cate line  of  light  extending  around  it.  In  some  cases  the 
time  of  internal  contact  at  egress  of  the  planet  was  missed, 
through  the  observer  mistaking  this  line  of  light  for  the  limb 
of  the  sun. 

Tliat  no  one  but  Rittenhouse  saw  this  line  of  lio-ht  duriuff 
the  transit  of  1769  is  to  be  attributed  to  tlie  low  altitude  of 
the  planet  at  most  of  the  stations,  and  to  the  imperfect  char- 
acter of  many  of  the  instruments  used.  It  is  also  to  be  re- 
marked that  the  observers  of  that  time  had  an  erroneous  no- 
tion of  the  appearance  which  would  be  presented  by  an  atmos- 
phere of  Venus.  It  was  supposed  that  the  atmosphere  would 
give  the  planet  a  nebulous  border  when  on  the  sun,  caused  by 
the  partial  absorption  of  the  light  in  passiug  through  it.  Cap- 
tain Cook,  at  Otaheite,  made  separate  observations  of  the 
contacts  of  the  supposed  atmosphere  and  of  the  planet  with 
the  limb  of  the  sun.  In  fact,  however,  it  would  not  be  possi- 
ble to  see  any  indications  of  an  atmosphere  under  such  cir- 
cumstances, for  the  reason  that  the  light  passing  through  its 
denser  portions  would  be  refracted  entirely  out  of  its  course, 
so  as  not  to  reach  an  observer  on  the  earth  at  all. 

The  spectroscope  shows  no  indication  that  the  atmosphere 
of  Venus  exerts  any  considerable  selective  absorption  upon 
the  light  which  passes  through  it.  No  new  and  well-marked 
spectral  lines  are  found  in  the  light  reflected  from  the  planet, 
nor  has  the  spectrum  been  certainly  found  to  differ  from  the 
regular  solar  spectrum,  except,  perhaps,  that  some  of  the  lines 
are  a  little  stronger.  This  would  indicate  that  the  atmosphere 
in  question  does  not  differ  in  any  remarkable  degree  from  our 
own,  or,  at  least,  does  not  contain  gases  which  exert  a  power- 
ful selective  absorption  on  light. 

Supposed  Visibility  of  the  Dark  Hemisphere  of  Venus. — Many 
astronomers  of  high  repute  have  seen  the  dark  atmosphere  of 
Venus  slightly  illuminated,  the  planet  presenting  the  appear- 
ance known  as  "  the  old  moon  in  the  new  moon's  arnis,'^  which 


296  THE  SOLAR  SYSTEM. 

may  be  seen  on  any  clear  evening  three  or  four  days  after  the 
cliange  of  the  moon.  It  is  well  known  that  in  the  case  of 
the  moon  her  dark  hemisphere  is  thus  rendered  visible  by  the 
light  retlected  from  the  earth.  But  in  the  case  of  Venus, 
there  is  no  earth  or  other  body  large  enough  to  shed  so  much 
light  on  the  dark  hemisphere  as  to  make  it  visible.  There 
being  no  sufficient  external  source  of  light,  it  has  been  attrib- 
uted to  a  phosphorescence  of  the  surface  of  the  planet.  If 
the  phosphorescence  were  always  visible  under  favorable  cir- 
cumstances, there  would  be  no  serious  difficulty  in  accepting 
this  explanation.  But,  being  only  rarely  seen,  it  is  hard  to 
conceive  how  any  merely  occasional  cause  could  act  all  at 
once  over  the  surface  of  a  planet  the  size  of  our  globe,  so  as 
to  make  it  shine.  Indeed,  one  circumstance  makes  it  ex- 
tremely difficult  to  avoid  the  conclusion  that  the  whole  ap- 
pearance is  due  to  some  unexplained  optical  illusion.  The 
appearance  is  nearly  always  seen  in  the  daytime  or  during 
bright  twilight — rarely  or  never  *ter  dark.  But  such  an  il- 
lumination would  be  far  more  easily  seen  by  night  than  by 
day,  because  during  the  day  an  appearance  easily  seen  at 
night  might  be  effaced  by  the  light  of  the  sky.  If,  then,  '^^e 
phenomenon  is  real,  why  is  it  not  seen  when  the  circumstances 
are  such  that  it  should  be  most  conspicuously  visible  ?  This 
is  a  question  to  which  no  satisfactory  answer  has  been  given, 
and  until  it  is  answered  we  are  justified  in  considering  the  ap- 
pearance to  be  purely  optical. 

Supposed  Satellite  of  Venus. — No  better  illustration  of  the  er- 
rors to  which  observations  with  imperfect  instruments  are  lia- 
ble can  be  given  than  the  supposed  observations  of  a  satellite 
of  Venus,  made  when  the  telescope  was  still  in  its  infancy. 
In  1672,  and  again  in  1686,  Cassini  saw  a  faint  object  near 
Venus  which  exhibited  a  phase  similar  to  that  of  the  planet. 
But  he  never  saw  it  except  on  these  two  occasions.  A  similar 
object  was  reported  by  Short,  of  England,  as  seen  by  him  on 
October  23d,  1740.  The  diameter  of  the  object  was  a  third 
of  that  of  Venus,  and  it  exhibited  a  similar  phase.  Several 
other  observers  saw  the  same  thing  between  1760  and  1764. 


THE  PLANET  VENUS.  297 

One  astronomer  went  so  far  as  to  compute  an  orbit  from  all 
the  observations ;  but  it  was  an  orbit  in  whicli  no  satellite  of 
Venus  could  possibly  revolve  unless  the  mass  of  the  planet  were 
ten  times  as  great  as  it  really  is.  A  century  has  now  elapsed 
without  the  satellite  having  been  seen,  and  the  fact  that  dur- 
ing this  century  the  planet  has  been  scrutinized  with  better 
telescopes  tlian  any  which  were  used  in  the  observations  re- 
ferred to  affords  abundant  proof  that  the  object  was  entirely 
mythical. 

How  the  observers  who  thought  they  saw  the  object  coulu 
liave  been  so  deceived  it  is  impossible,  at  this  distance  of 
time,  to  say  with  certainty.  Had  they  been  inexperienced, 
we  could  say  with  some  confidence  that  they  were  mir-led  by 
the  false  images  produced  to  some  extent  in  every  *  escopo 
by  the  light  reflected  from  the  cornea  of  the  eye  against  the 
nearest  surface  of  the  eye-piece,  and  thence  back  again  into 
the  e3'e.  Similar  images  are  sometimes  produced  by  the  re- 
flection of  liffht  between  the  surfaces  of  the  various  lenses  of 
the  eye  -  piece.  They  are  well  known  to  astronomers  under 
the  name  of  "  ghosts ;"  and  one  of  tlie  first  things  a  young  ob- 
server must  learn  is  to  distinguish  them  from  real  objects. 
They  may  also  arise  from  a  slight  maladjustment  of  the  lenses 
of  the  eye-piece,  and  if,  proceeding  from  this  cause,  they  are 
produced  only  when  the  actual  object  is  in  the  centre  of  the 
field,  they  may,  for  the  moment,  deceive  the  most  experienced 
observer.*  If,  in  an  ordinary  achromatic  telescope,  in  which 
the  interior  curvatures  of  the  lenses  are  the  same,  the  latter 
arc  not  exactly  at  the  same  distance  all  the  way  round,  a  ghost 
will  be  seen  along-side  of  every  bright  object  in  all  positions. 
It  is  probable  that  all  the  observations  alluded  to  were  the  re- 
sults of  some  sort  of  derangements  in  the  telescope,  producing 
false  images  by  reflection  from  the  glasses. 

*  One  of  the  eye-pieces  of  the  great  Washington  telescope  shows  a  beautiful 
little  satellite  along -side  the  planet  Uranus  or  Neptune  when  tlie  image  of  the 
planet  is  brought  exactly  in  the  centre  of  the  field  of  view,  but  it  disappears  as 
soon  as  t'.e  telescope  is  moved.  The  writer  was  deceived  by  this  appearance  on 
two  occasions  while  scrutinizing  these  planets  for  close  satellites. 


298  THE  SOLAR  SYSTEM. 

%  4.  The  Earth. 

Our  earth  is  the  third  planet  in  the  order  of  distance  from 
the  sun,  and  slightly  the  largest  of  the  inner  group  of  four. 
Its  mean  distance  from  the  sun  is  about  92^  millions  of  miles ; 
but  it  is  a  million  and  a  half  less  than  this  mean  on  January 
1st  of  every  year,  and  as  much  greater  on  July  1st.  That 
is,  its  actual  distance  varies  from  91  to  94  millions  of  miles. 
As  already  remarked,  these  numbers  are  uncertain  by  several 
hundred  thousand  miles. 

Much  of  what  we  may  call  the  astronomy  of  the  earth — 
such  as  its  figure  and  mass,  the  length  of  the  year,  the  obliq- 
uity of  the  ecliptic,  the  causes  of  the  changes  in  the  seasons 
and  in  the  length  of  the  days — has  already  been  treated  in 
the  chapter  on  gravitation,  so  that  we  have  little  of  a  purely 
astronomical  character  to  add  here.  The  features  of  its  sur- 
face and  the  phenomena  of  its  atmosphere  belong  rather  to 
geography  and  meteorology  than  to  astronomy.  But  its  consti- 
tution gives  rise  to  several  questions  in  the  treatment  of  which 
astroiiomical  considerations  come  into  play.  Prominent  among 
these  is  that  of  the  state  of  the  great  interior  mass  of  our 
globe,  whether  solid  or  liquid.  It  is  well  known  that  wher- 
ever we  descend  into  the  solid  portions  of  tlie  earth,  we  find  a 
rise  in  temperature,  going  on  uniformly  with  the  depth,  at  a 
rate  which  nowhere  differs  greatly  from  1°  Fahrenheit  in  50 
feet.  This  rise  of  temperature  has  no  connection  with  the 
sea-level,  but  is  found  at  all  points  of  the  surface,  no  matter 
how  elevated  they  may  be.  Wherever  a  difference  of  temper- 
ature like  this  exists,  there  is  no  ^ssarily  a  constant  transfer  of 
heat  from  the  warmer  to  the  cooler  strata  by  conduction.  In 
this  way,  the  inequality  would  soon  disappear  by  the  warmer 
strata  cooling  off,  if  there  were  not  a  constant  supply  of  heat 
inside  the  earth.  The  rise  of  temperature,  therefore,  cannot 
be  something  merely  superficial,  but  must  continue  to  a  great 
depth.  If  we  trace  to  past  times  the  conditions  which  must 
have  existed  in  order  that  the  increase  mi^ht  show  itself  at  the 
present  time,  we  shall  find  it  almost  certain  that,  a  thousand 


THE  EABTH.  299 

j^ears  ago,  the  whole  eartli  was  red-hot  at  a  distance  of  ten  or 
fifteen  miles  below  its  surface;  because  cJierwise  its  interior 
could  not  have  furnished  the  supply  of  heat  which  now  causes 
the  observed  increase.  This  being  the  case,  it  is  probably  red- 
hot  still,  since  it  would  be  absurd  to  expect  a  state  of  things 
like  this  to  be  merely  temporary.  In  a  word,  we  have  every 
reason  to  believe  that  the  increase  of  say  100°  a  mile  contin- 
ues many  miles  into  the  interior  of  the  earth.  Then  we  shall 
have  a  red  heat  at  a  distance  of  12  miles,  while,  at  the 
depth  of  100  miles,  the  temperature  will  be  so  high  as  to 
melt  most  of  the  materials  which  form  the  solid  crust  of  the 
globe. 

We  are  thus  led  to  the  theory,  very  generally  received  by 
geologists,  that  the  earth  is  really  a  sphere  of  molten  matter 
surrounded  by  a  comparatively  thin  solid  crust,  on  which  we 
live.  This  crust  floats,  as  it  were,  on  the  molten  interior.  It 
must  be  confessed  that  geological  facts  are,  on  the  whole,  fa- 
vorable to  this  view.  Observations  on  the  pendulum  have 
been  supposed  to  show  that  the  specific 
gravity  of  the  earth  under  the  great 
mountain  chains  is  generally  less  than  in 
the  adjoining  plains,  which  is  exactly  the 
result  that  would  flow  from  the  theory. 
The  heavier  masses,  pressing  upon  the  in- 
terior fluid,  would  tend  to  elevate  the  sur- 
rounding lighter  masses,  and  when  the  two  F10.75.— showing  thickness 
were  in  equilibrium,  tl,e  latter  would  be  tX^^liS:^^ 
the  higher,  as   a  floating  block  of  pine     ry  of  a  molten  interior. 

ml  .    1  i        r    ii  .  The  circle  is  thicker  in 

nse  higher  out  of  the  water     proportion  than  the  solid 

than  a  block  of  oak.  Boiling  springs  in  *=™^'- 
many  parts  of  the  globe  show  that  there  are  numerous  hot  re- 
gions in  the  earth's  interior,  and  this  heat  cannot  be  merely 
local,  because  then  it  would  soon  be  dissipated.  But  the  geol- 
ogist finds  the  strongest  proof  of  the  theory  in  volcanoes  and 
earthquakes.  The  torrents  of  lava  which  have  been  thrown 
out  of  the  former  through  thousands  of  years  show  that  there 
are  great  volumes  of  molten  matter  in  the  earth's  interior, 


300  THE  SOLAR  SYSTEM. 

while  the  latter  show  this  interior  to  be  subject  to  violent 
changes  which  a  solid  could  not  exhibit. 

But  mathematicians  have  never  been  able  entirely  to  rec- 
oncile the  theory  in  question  with  the  observed  phenomena  of 
precession,  nutation,  and  tides.  To  all  appearance,  the  earth 
resists  the  tide-producing  action  of  the  sun  and  moon  exactly 
as  if  it  were  solid  from  centre  to  circumference.  Sir  William 
Thomson  has  shown  that  if  the  earth  were  less  rigid  than  steel, 
it  would  yield  so  much  to  this  action  that  the  tides  would  be 
much  smaller  than  on  a  perfectly  rigid  earth ;  that  is,  the  at- 
traction of  the  bodies  in  question  would  draw  the  earth  itself 
out  into  an  ellipsoidal  form,  instead  of  drawing  merely  the 
waters  of  the  ocean.  Earth  and  ocean  moving  together,  we 
could  see  no  tides  at  all.  If  the  earth  were  only  a  thin  shell 
floating  on  a  liquid  interior,  the  tides  would  be  produced  in 
the  latter ;  the  thin  shell  would  bend  in  such  a  way  that  the 
tides  in  the  ocean  would  be  nearly  neutralized.  Again,  the 
question  has  arisen  whether  the  liquid  interior  would  be  af- 
fected by  precession  ;  whether,  in  fact,  the  crust  would  not  slip 
over  it,  so  that  in  time  the  liquid  would  rotate  in  one  direc- 
tion, and  the  crust  in  another.  Altogether,  the  doctrine  of  the 
earth's  fluidity  is  so  fraught  with  difliculty  that,  notwithstand- 
ing the  seeming  strength  of  the  evidence  in  its  favor,  it  must 
be  regarded  as  at  least  very  doubtful.  It  may  be  added  that 
no  one  denies  that  the  interior  of  our  planet  is  intensely  hot — 
hot  enough,  in  fact,  to  melt  the  rocks  at  i<^s  curface  —  but  it 
is  supposed  that  the  enormous  pressure  of  the  outer  portions 
tends  to  keep  the  inner  part  from  melting.  Nor  is  it  ques- 
tioned by  Sir  William  Thomson  that  there  are  great  volumes 
of  melted  nuitter  'n  the  earth's  interior  from  wluJi  volcanoes 
are  fed;  but  he  maintains  that,  after  all,  these  volumes  are 
small  compared  with  that  of  the  whole  earth. 

Refraction  of  the  Atmospliere. — If  a  ray  of  light  pass  through 
our  atmosphere  in  any  other  than  a  vertical  direction,  it  is 
constantly  curved  downwards  by  the  refractive  power  of  that 
medium.  The  more  nearly  horizontal  the  course  of  the  ray, 
the  greater  the  curvature.      In  consequence  of  this,  all  the 


TEE  EARTH.  301 

heavenly  bodies  appear  a  little  nearer  the  zenith,  or  a  little 
higher  above  the  horizon,  than  they  actually  are.  The  dis- 
placement is  too  small  to  be  seen  by  the  naked  eye  except 
quite  near  the  horizon,  where  it  increases  rapidly,  amounting 
to  more  than  half  a  degree  at  the  horizon  itself.  Consequent- 
ly, at  any  point  where  we  have  a  clear  horizon,  as  on  a  prairie, 
or  the  sea-shore,  the  whole  disk  oi  the  sun  will  be  seen  above 
the  horizon  when  the  true  direction  is  below  it.  A  slight  in- 
crease is  thus  given  to  tlie  length  of  the  day.  The  sun  in  our 
latitudes  always  rises  three  or  four  minutes  sooner,  and  sets 
three  or  four  minutes  later,  than  he  would  if  there  were  no 
atmosphere.  At  the  time  of  the  equinoxes,  if  we  suppose  the 
day  to  begin  and  end  when  the  centre  of  the  sun  is  on  die 
horizon,  it  is  not  of  the  same  length  with  the  night,  but  is  six 
or  eight  minutes  longer.  If  we  suppose  the  day  to  begin  with 
the  rising  of  the  sun's  upper  limb,  and  not  to  end  till  the  same 
limb  has  set,  then  we  must  add  some  three  minutes  more  to 
its  length. 

If,  standing  on  a  hill,  we  watch  the  sun  rise  or  set  over  the 
ocean,  one  effect  of  refraction  will  be  quite  clearly  visible. 
When  his  lower  limb  almost  seems  to  touch  the  water,  it  will 
be  seen  that  the  form  of  his  disk  is  no  longer  round,  but  ellip- 
tical, the  horizontal  diameter  being  greater  than  the  vertical. 
The  reason  of  this  is  that  the  lower  limb  is  more  elevated  by 
refi'action  than  the  upper  one,  and  thus  the  vertical  diameter 
is  diminished. 

In  practical  astronomy,  all  observations  of  the  altitude  of 
the  heavenly  bodies  above  the  horizon  must  be  corrected  for 
refraction,  the  true  altitude  being  always  less  than  that  ob- 
served. Very  near  the  zenith  the  refract'on  is  about  1"  for 
every  degree,  or  TrirV^r  V^^'^  ^^^®  distafice  from  the  zenith.  But 
it  increases  at  first  in  the  proportion  of  the  tangent  of  the  ze- 
nith distance,  so  that  at  45°,  or  half-way  between  the  zenith 
and  the  horizon,  it  amounts  to  60" ;  at  the  horizon  it  is  34'. 

IVie  Aurora  Borealis. —  This  phenomenon,  though  so  well 
known,  is  one  of  which  great  difiiculty  has  been  found  in  giv- 
ing a  satisfactory  explanation.     That  it  is  in  some  way  con- 


302 


THE  SOLAR  SYSTEM. 


Fio.  70.— Distribution  of  iiurorae,  nfter  Lootnin.    Tiie  darker  the  color,  the  more  frequently 

auroras  are  f*een. 

nected  with  the  jiole  of  tlie  earth  is  sliown  by  tlie  fact  tliat 
its  frcqiiCDcy  depends  on  the  latitude.  In  the  equatorial  re- 
gions of  our  globe  it  is  quite  rare,  and  increases  in  frequency 
as  we  go  north.     But  the  region  of  greatest  frequency  seems 


THE  EARTH. 


303 


to  be,  not  the  poles,  but  the  neighborhood  of  the  Arctic  Cir- 
cle, from  which  it  diminishes  towards  both  the  north  and  the 
south.  This  is  shown  more  exactly  in  Professor  Loomis's 
auroral  map,  of  which  we  give  a  copy  on  the  preceding  page. 
A  close  study  of  the  aurora  indicates  that  its  connection  is 
not  with  the  geographical,  but  with  the  maguetic  pole.  Two 
distinct  kinds  of  light  are  seen  in  the  aurora;  or  we  might 
say  that  the  light  assumes  two  distinct  forms,  of  which  some- 
times the  one  and  sometimes  the  other  preponderates.  They 
are  as  follows : 

1.  The  cloud-like  form.  This  consists  of  a  large  irregular 
patch  of  light,  frequently  of  a  red  or  purple  tinge.  It  is  seen 
in  every  direction,  but  more  frequently  in  or  near  the  northern 
horizon,  where  it  assumes  the  form  of  an  arch  or  crown  of 
light.  The  two  ends  of  the  arch  rest  on  the  horizon,  one  on 
each  side  of  the  north  point.  The  middle  of  the  arch  rises  a 
few  deijrrces  above  the  horizon. 


Fid.  77. — View  of  uurorii. 


2.  The  streamer  or  pillar  form.  This  form  consists  of  long 
streamers  or  pillars,  which  extend  in  the  direction  of  the  dip- 
ping magnetic  needle.  They  look  curved  or  archod,  like  the 
celestial  sphere  on  which  they  are  projected,  but  they  are  re- 
ally straight.    They  are  iu  a  state  of  constant  motion.    Some- 


304  THE  SOLAR  SYSTEM. 

times  they  are  spread  out  in  the  form  of  an  immense  flag 
with  numerous  folds,  dancing,  quivering,  and  undulating,  as 
if  moved  by  the  wind. 

Electric  Nature  of  the  Aurora. — There  is  abundant  evidence 
that  tlie  aurora  is  intimately  connected  with  the  electricity 
and  magnetism  of  the  earth.  During  a  brilliant  aurora  such 
strong  and  irregular  currents  of  electricity  pass  through  the 
telegraph  wires  that  it  is  difficult  to  send  a  despatch.  Some- 
times the  current  runs  with  such  force  that  a  messacre  mav 
be  sent  without  a  battery.  The  magnetic  needle  is  also  in  a 
state  of  great  agitation.  Before  the  spectroscope  came  into 
use,  these  electric  phenomena  gave  rise  to  the  opinion  that 
the  aurora  was  due  entirely  to  currents  of  electricity  passing 
through  the  upper  regions  of  the  atmosphere  from  one  pole  to 
the  other.  But  recent  researches  seem  to  show  that,  though 
this  view  may  be  partly  true,  it  is  far  from  the  whole  truth, 
and  does  not  afford  a  complete  explanation.  The  great  height 
of  the  aurora  and  the  nature  of  its  spectrum  both  militate 
against  it. 

He  i(j] it  of  the  Aurora. — Several  attempts  have  been  made  in 
recent  times  to  determine  the  height  of  the  aurora  above  the 
surface  of  the  earth,  by  simultaneous  observations  of  some 
prominent  streamer  or  patcii  of  light  from  several  far-distant 
stations.  The  general  result  is  that  it  extends  to  the  height  of 
from  400  to  600  miles.  But  the  evidence  of  shooting -stars 
and  meteors  seems  to  indicate  that  the  limit  of  the  atmosphere 
is  between  100  and  110  miles  in  height.  If  it  extends  above 
this,  it  must  be  too  rare  to  conduct  electricity  long  before  it 
reaches  the  greatest  height  of  the  aurora  ;  indeed,  it  is  doubt- 
ful whether  it  does  not  attain  this  rarity  at  a  heiglit  of  40  or 
50  miles.  If,  then,  the  aurora  really  extends  to  the  great 
height  we  have  mentioned,  and  still  exists  in  a  gaseous  medi- 
um, it  seems  difficult  to  avoid  the  conclusion  that  this  medium 
is  something  far  more  ethereal  than  tlio  gases  which  form  our 
atmosphere.  It  would,  however,  be  uni)hil()sophical  to  assume 
the  existence  of  such  a  medium  without  some  other  evidence 
in  its  favor  than  that  afforded  by  the  aurora.     We  must  in- 


THE  EARTH. 


3U5 


elude  tlie  aurora  amonsj  those  thiuo;s  in  wliich  inoderu  ob- 
servations  have  opened  up  more  difticulties  than  modern  theo- 
ries have  explained. 

Spectrum  of  tJie  Aurora. — The  spectrum  of  the  aurora  is  so 
far  from  uniform  as  to  be  quite  puzzling.  There  is  one  cliar- 
aeteristie  bright  line  in  the  green  i)art  of  the  spectrum,  known 
as  Angstrom's  line,  from  its  first  discoverer.  This  was  the 
only  line  Angstrom  coidd  see:  he  therefore  pronounced  the 
light  of  the  aurora  to  be  entirely  of  one  color.  Subsecpient 
observers,  however,  saw  many  additional  lines,  but  they  were 
different  in  different  auroras.  Among  those  who  have  made 
careful  studies  of  the  aui'ora  with  the  s})ectroscope  are  the 
late  Professor  Winlock,  of  Harvard  University;  Professor 
Barker,  of  Philadelphia ;  and  Dr.  II.  C.  Vogel,  formerly  of 
Bothkamp. 


n  V.      h  F 

Fio.  'S.— Spectrum  of  two  of  the  great  auroras  of  18T1,  after  Dr.  II.  C.  VogeL 

Fig.  78  shows  the  spectra  of  two  auroras,  as  drawn  by  Dr. 
Vogel.  It  will  be  seen  that  there  is  one  fine  bright  line  be- 
tween D  and  /i',  which  woidd  fall  in  the  yellowish-green  part 
of  the  spectrum,  while  the  others  are  all  broad,  ill -defined 
bands.  Dr.  A^ogel  notices  a  remarkable  connection  between 
these  lines  and  several  groups  of  lines  produced  by  the  vapor 
of  iron,  and  inquires  whether  this  vapor  can  possibly  exist  in 
the  upper  regions  of  our  atmosphere.  A  more  complete  study 
of  the  spectra  of  vapors  at  different  pressures  and  tempera- 
tures is  necessary  before  we  can  form  a  decided  opinion  as  tO' 
\vliat  the  aurora  reallv  is. 

21 


306 


THE  SOLAR  SYSTEM. 


Of  the  supposed  periodicity  of  the  aurora,  and  its  connection 
with  sun-spots,  we  liave  ah'cady  spoken.  Granting  tlie  reality 
of  this  connection,  we  may  expect  that  auroras  will  be  very 
frequent  between  the  years  1880  and  1884;  and  if  this  ex- 
pectation is  realized,  little  doubt  of  the  connection  will  remain. 

§  5.  The  Moon. 

The  moon  is  much  the  nearest  to  us  of  all  the  heavenly 
bodies ;  no  other,  except  possibly  a  comet,  ever  coming  nearer 
than  a  hundred  times  her  distance.  Her  mean  distance  is,  in 
round  numbers,  240,000  miles.  Owing  to  the  ellipticity  of  her 
orbit  and  the  attractive  force  of  the  eun,  it  varies  from  ten  to 
twenty  thousand  miles  on  each  side  of  this  mean  in  the  course 
of  each  monthly  revolution.  The  least  possible  distance  is 
221,000  miles ;  the  greatest  is  259,600  miles.  It  very  rarely 
approaches  either  of  these  limits,  the  usual  oscillation  being 
about  13,000  miles  on  each  side  of  the  mean  distance  of 
240,300.  The  diameter  of  the  moon  is  2160  miles,  or  some- 
what less  than  two-sevenths  that  of  the  earth.  Her  volume  is 
about  one-fiftieth  that  of  the  earth,  and  if  she  were  as  dense 
as  the  latter,  her  mass  would  be  in  the   same  proportion. 


Fio.  79.— Relative  size  of  eaith  and  moon. 


But  her  actual  mass  is  only  about  one-eightieth  that  of  the 
earth,  showing  that  her  density,  or  the  specific  gravity  of  the 
material  of  which  she  is  composed,  is  little  more  than  half  that 


THE  MOON.  307 

of  onr  globe.  Her  weight  is,  in  fact,  about  3|^  times  that  of 
her  bulk  of  water. 

The  most  remarkable  feature  of  the  motion  of  the  moon  is, 
that  she  makes  one  revolution  on  her  axis  in  the  same  tiniD 
that  she  revolves  around  the  earth,  and  so  always  presents  tlie 
same  face  to  us.  In  consequence,  the  other  side  of  the  moon 
must  remain  forever  invisible  to  human  eyes.  The  reason  of 
this  peculiarity  is  to  be  found  in  the  ellipticity  of  her  globe. 
Tliat  slie  should  originally  have  been  set  in  revolution  on  her 
axis  with  precisely  the  same  velocity  with  which  she  revolved 
around  the  earth,  so  that  not  the  slightest  variation  in  the  re- 
lation of  the  two  motions  should  ever  occur  in  the  course  of 
ages,  is  highly  improbable.  If  such  had  been  the  state  of 
things,  the  correspondence  of  the  two  motions  could  not  have 
been  kept  up  without  her  axial  rotation  varying;  because, 
owing  to  the  secular  acceleration  already  described,  the  moon, 
in  the  course  of  ages,  varies  her  time  of  revolution,  and  so 
the  two  motions  would  cease  to  correspond.  But  the  effect  of 
the  attraction  of  the  earth  upon  the  slightly  elongated  lunar 
globe  is  such  that  if  the  two  motions  are,  in  the  beginning, 
very  near  together,  not  only  will  the  axial  rotation  accommo- 
date itself  to  the  orbital  revolution  around  the  earth,  but  as 
the  latter  varies,  the  former  will  vary  with  it,  and  thus  the 
correspondence  will  be  kept  up. 

Figure^  Rotation^  and  Libration  of  the  Afoon. — Supposing  the 
shape  of  the  moon  to  be  the  same  as  if  it  were  a  fluid  mass, 
or  covered  by  an  ocean,  it  will  be  an  ellipsoid  with  three  un- 
equal axes.  The  shortest  axis  will  be  that  around  which  it 
revolves,  which  is  not  very  far  from  being  perpendicular  to 
the  ecliptic.  The  next  longest  is  that  which  lies  in  the  direc- 
tion in  which  the  moon  moves ;  while  the  longest  of  all  is 
that  which  points  towards  the  earth.  The  reason  that  the 
polar  axis  is  the  shortest  is  the  same  which  makes  the  polar 
axis  of  the  earth  the  shortest,  tliat  is,  the  centrifugal  force 
generated  by  the  revolution  round  that  axis.  If  we  consid- 
ered only  the  action  of  this  force,  we  should  conclude  that  the 
jnoon,  like  the  earth,  was  an  oblate  spheroid,  the  equator  be- 


30S  THE  SOLAR  SYSTEM. 

ing  a  perfect  circle.  But  the  attraction  of  the  earth  upon  the 
moon  tends  to  elongate  it  in  the  direction  of  the  line  joining 
the  two  bodies,  in  the  same  way  tliat  the  atti'action  of  the  moon 
upon  the  earth  generates  a  tide-producing  force  which  we  have 
already  explained.  At  the  centre  of  the  moon  the  attraction 
of  the  earth  and  the  centrifugal  force  of  the  moon  in  its  or- 
bit exactly  balance  each  other.  But  if  we  go  to  the  farther 
side  of  the  moon,  the  centrifugal  force  will  be  greater,  owing 
to  the  larger  orbit  whicli  that  part  of  the  moon  has  to  de- 
scri])e,  while  the  attraction  of  tlie  earth  will  be  less  owing  to 
the  greater  distance  of  the  particles  it  attracts.  Hence,  that 
part  of  the  moon  tends  to  fly  off  from  the  centre  and  from  the 
earth.  On  this  side  of  the  moon  the  case  is  reversed,  the  at- 
tractive force  of  the  earth  exceeding  the  centrifugal  force  of 
those  parts  of  the  moon,  whence  those  parts  are  impelled  by  a 
force  tending  to  draw  tiiem  to  the  earth.  The  effect  would 
be  much  the  same  as  if  a  rope  were  fastened  to  this  side  of 
the  moon,  and  constantly  pulled  towards  the  earth,  while  an- 
other were  fastened  to  the  op})osite  side,  and  as  constantly 
pulled  from  the  earth.  Supposing  the  moon  to  be  a  liquid, 
so  as  to  yield  freely,  it  is  clear  that  the  effect  of  these  forces 
would  be  to  elongate  her  in  the  direction  of  the  earth. 

The  deviations  from  a  spherical  form  produced  by  these 
causes  are  very  minute.  Taking  the  results  of  Lagrange  and 
Newton,  the  mean  axis  would  be  46^  feet  longer  than  the 
shortest  one,  and  the  longest  186  feet  lonsrer  than  the  mean 
one,  or  232^  feet  longer  than  the  shortest  one.*  These  differ- 
ences are  so  much  smaller  than  the  average  height  of  the 
lunar  mountains  that  the  irregularities  produced  by  the  latter 
might  entirely  overpower  them ;  but  the  correspondence  be- 
tween the  motions  of  rotation  and  revolution  of  the  moon 
shows  that  there  must  be,  on  the  average,  a  real  elongation  in 

*  These  numbers  are,  perliaps,  not  strictly  correct.  The  extension  of  186  feet 
wns  deduced  by  Newton  from  a  comparison  of  the  distorting  powers  of  tiie  centrif- 
ugal force  of  the  earth  with  that  of  the  force  we  have  just  described.  He  seems 
to  liave  overlooked  the  fact  that  the  small  density  of  the  moon  will  cause  the 
elongation  to  be  greater. 


THE  MOON.  309 

the  .direction  of  tlio  earth.  This  correspondence  is  kept  up  by 
the  slight  additional  attraction  of  the  earth  upon  this  extension 
of  the  moon  towards  the  earth,  combined  with  the  additional 
centrifugal  force  of  the  extension  on  the  other  side.  Although 
these  forces  are  not  by  any  means  the  same  as  the  distorting 
forces  already  described,  tliey  may  be  re])resented  in  the  same 
M-ay  by  two  ropes,  one  of  which  pulls  the  protuberance  on  this 
side  towards  the  earth,  while  the  other  pulls  the  protuberance 
on  the  other  side  from  it.  If  the  two  protuberances  do  not 
point  exactly  towards  the  earth,  the  effect  of  these  two  minute 
forces  will  be  to  draw  them  very  slowly  into  line.  Conse- 
quently, notwithstanding  the  slow  variations  to  wliicli  the  mo- 
tion of  the  moon  around  the  earth  is  subject  in  the  course 
of  ages,  the  attraction  of  the  earth  will  always  keep  this  ])ro- 
tuberaut  face  turned  towards  us.  Human  eyes  will  never  be- 
hold the  other  side  of  the  moon,  unless  some  external  force 
acts  upon  her  so  as  to  overcome  the  slight  balancing  force 
just  described,  and  set  her  in  more  or  less  rapid  motion  on 
her  axis.  If  it  is  disappointing  to  reflect  that  we  are  for- 
ever deprived  of  the  view  of  the  other  side  of  our  satellite,  we 
may  console  ourselves  with  the  reflection  that  there  is  not  the 
slightest  reason  to  believe  that  it  differs  in  any  respect  from 
this  side.  The  atmosphere  with  which  it  has  been  covered, 
and  the  inhabitants  with  which  it  has  been  peopled,  are  no 
better  than  the  products  of  a  poetic  imagination. 

The  forces  we  have  just  described  as  tending  to  keep  the 
same  face  of  the  moon  pointed  towards  us  would  not  produce 
this  effect  unless  the  adjustment  of  the  two  motions — that 
around  the  earth,  and  that  on  her  axis — were  almost  perfect 
in  the  beijinnino;.  If  her  axial  rotation  were  accelerated  by  so 
small  an  amount  as  one  revolution  in  two  or  three  years,  there 
is  every  reason  to  believe  that  she  would  keep  on  revolving  at 
the  new  rate,  notwithstanding  the  force  in  question.  The  case 
is  much  like  that  of  a  very  easy-turning  fly-wheel,  which  is 
slightly  weighted  on  one  side.  If  we  give  the  wheel  a  gentle 
motion  in  one  direction  or  another,  the  weight  will  cause  the 
wheel  to  turn  till  the  heavy  side  is  the  lowest,  and  the  wheel 


310  THE  SOLAR  SYSTEM. 

will  then  vibrato  very  slowly  on  one  side  and  the  other  of  this 
point.  But  if  we  give  the  wheel  a  motion  rapid  enough  to 
carry  its  heavy  side  over  the  highest  point,  then  the  weight 
will  accelerate  the  wheel  while  it  is  falling  as  nuich  as  it  will 
retard  it  while  rising ;  and  if  there  were  no  friction,  the  wheel 
would  keep  on  turning  indefinitely.  The  question  now  arises. 
How  does  it  happen  that  these  two  motions  are  so  exactly  ad- 
justed to  each  other  that  not  only  is  the  longer  axis  of  the 
moon  pointed  exactly  towards  the  earth,  but  not  the  slightest 
swing  on  one  side  or  the  other  can  be  detected  ?  That  this 
adjustment  should  be  a  mere  matter  of  chance,  without  any 
physical  cause  to  produce  it,  is  almost  infinitely  improbable, 
while  to  suppose  it  to  result  from  the  mere  arbitrary  will  of 
the  Creator  is  contrary  to  all  scientific  philosophy.  But  if  the 
moon  were  once  in  a  partially  fluid  state,  and  rotated  on  her 
axis  in  a  period  different  from  her  present  one,  then  the  enor- 
mous tides  produced  by  tlie  attraction  of  the  earth,  combined 
with  the  centrifugal  force,  would  be  accompanied  by  a  fric- 
tion which  would  gradually  retard  the  rate  of  rotation,  until 
it  was  reduced  to  the  point  of  exact  coincidence  with  the  rate 
of  revolution  round  the  earth,  as  we  now  find  it.  We  there- 
fore see  in  the  present  state  of  things  a  certain  amount  of 
probable  evidence  that  the  moon  was  once  in  a  state  of  par- 
tial fluidity. 

The  force  we  have  just  described  as  drawing  the  protuber- 
ant portion  of  the  moon  towards  the  earth  is  so  excessively 
minute  that  it  takes  it  a  long  time  to  produce  any  sensible  ef- 
fect ;  consequentl}',  although  the  moon  moves  more  rapidly  in 
some  points  of  her  orbit  than  in  others,  the  force  in  question 
produces  no  corresponding  change  in  the  moon's  rotation. 
The  protuberance  does  not,  therefore,  always  point  exactly  at 
the  earth,  but  sometimes  a  little  one  side,  and  sometimes  a  lit- 
tle the  other,  according  as  the  moon  is  ahead  of  or  behind  her 
mean  place  in  the  orbit.  The  result  is,  that  tlie  face  which 
the  moon  presents  to  us  is  not  always  exactly  the  same,  there 
being  a  slight  ajjparent  (not  real)  oscillation,  due  to  the  real 
inequality  in  her  orbital  motion.     This  apparent  sw^aying  is 


THE  MOON.  311 

called  Uhration,  and  in  consequence  of  it  there  is  nearly  six- 
tenths  of  the  hmar  surface  wiiich  may,  at  one  time  or  another, 
come  iuto  view  from  the  earth. 

llie  Lunar  Day. — In  -"onsequence  of  the  peculiarity  in  the 
moon's  rotation  whicli  we  have  described,  the  hmar  day  is  29^ 
times  as  long  as  tlie  terrestrial  day.  Near  tlie  moon's  equator 
the  sun  shines  without  intermission  nearly  fifteen  of  our  days, 
and  is  absent  for  the  same  length  of  time.  In  consequence, 
the  vicissitudes  of  temperature  to  which  the  surface  is  exposed 
must  be  very  great.  During  the  long  lunar  night  the  temper- 
ature of  a  body  on  the  moon's  surface  would  probably  fall 
below  any  degree  of  cold  that  we  ever  experience  on  the  earth, 
while  during  the  day  it  nmst  become  liotter  than  anywhere 
on  our  globe. 

Astronomical  phenomena,  to  an  observer  on  the  moon,  would 
exhibit  some  peculiarities.  The  earth  would  be  an  immense 
moon,  going  through  the  same  phases  that  the  moon  does  to 
us ;  but  instead  of  rising  and  setting,  it  would  only  oscillate 
back  and  forth,  through  a  few  degrees.  On  the  other  side  of 
the  moon  it  would  never  be  seen  at  all.  The  diurnal  motion 
of  the  stai-s  would  take  place  in  twentj^- seven  of  our  days, 
much  as  they  do  here  every  day,  while,  as  we  have  said,  the 
sun  would  rise  and  set  in  29|^  of  our  days. 

Geography  of  the  Moon.  —  With  the  naked  eye  it  is  quite 
readily  seen  that  the  brilliancy  of  the  moon  is  far  from  uni- 
form, her  disk  being  variegated  with  irregular  dark  patches, 
which  have  been  supposed  to  bear  a  rude  resemblance  to  a 
human  face.  It  is  said  to  have  been  a  fancy  of  some  of  the 
ancient  philosophers  that  the  light  and  dark  portions  were 
caused  by  the  reflection  of  the  seas  and  continents  of  the  ter- 
restrial globe,  though  it  is  hard  to  conceive  of  such  an  o[)in- 
ion  being  seriously  entertained.  The  first  rude  idea  of  the 
real  nature  of  the  lunar  surface  was  gained  by  Galileo  with 
his  telescope.  He  saw  that  the  brighter  portions  of  the  disk 
were  broken  up  with  inequalities  of  the  nature  of  mountains 
and  craters,  while  tlie  dark  parts  were,  for  the  most  part, 
smooth  and  uniform.     Here  he  saw  a  striking  resemblance  to 


312  THE  SOLAR  SYSTEM. 

the  geograpliical  features  of  our  globe,  and  is  said  to  have  sug- 
gested that  the  brighter  and  rougher  portions  might  be  conti- 
nents, and  the  dark,  smooth  portions  oceans.  This  view  of  the 
resemblance  to  terrestrial  scenery  is  commemorated  in  Mil- 
ton's description  of  Satan's  shield : 

"  Like  the  moon,  whose  orb 
Through  optic  ghiss  the  Tuscan  artist  views 
At  evening,  from  the  top  of  Fesole', 
Or  in  ^/^aldarno,  to  descry  new  hinds, 
Kivers,  or  mountains  in  her  spotty  globe." 

The  opinion  that  the  dark  portions  of  the  lunar  disk  were 
seas  was  shared  by  Kepler,  Ilevelius,  and  Ricciohis.  The  last 
two  made  maps  of  the  moon  in  which  they  gave  names  to  the 
supposed  seas,  which  names  the  regions  still  bear,  though  they 
are  strikingly  fanciful.  Among  them  are  Oceanus  Procella- 
mm  (the  Ocean  of  Storms),  Mare  Tranquillllatis  (Sea  of  Tran- 
(juillity),  Mare  Imhrium  (Rainy  Sea),  etc.  The  names  of  great 
philosophers  and  astronomers  were  given  to  prominent  feat- 
ui'es,  craters,  etc. 

If  this  resemblance  between  the  earth  and  moon  had  been 
established ;  if  it  had  been  found  that  our  satellite  really  had 
seas  and  atmosphere,  and  was  fitted  for  the  support  of  or- 
ganic life;  still  more,  if  any  evidence  of  the  existence  of  in- 
telligent beings  had  been  found,  our  interest  in  lunar  geogra- 
phy would  have  been  immensely  heightened.  But  the  more 
the  telescope  was  improved,  the  more  clearly  it  was  seen  that 
there  was  no  similarity  between  lunar  and  terrestrial  scenery. 
A  very  slight  increase  of  telescopic  power  showed  that  there 
was  no  more  real  smoothness  in  the  regions  of  the  sujiposed 
seas  than  elsewhere.  The  ine(pialities  .vere  smaller  and  hard- 
er to  see  on  account  of  the  darkness  of  color ;  but  that  was 
all.  The  sun  would  have  been  brilliantly  imaged  back  from 
the  surfaces  of  the  oceans  in  certain  positions  of  the  moon ; 
but  nothing  of  the  kind  was  ever  seen.  The  polariscopo 
showed  that  the  sun's  rays  did  not  pass  through  any  licpiid  at 
the  moon's  surface.  Positive  evidence  of  an  atmosphere  was 
souglit  in  vain.     Supposed  volcanoes  were  traced  to  bright 


THE  MOON, 


313 


spots,  illuminated  by  light  from  the  earth.  Inequalities  of 
surface  there  were ;  but  in  form  they  were  wholly  different 
from  the  mountains  of  the  earth.     So  the  beautiful  fancies  of 


Fig.  S(» View  of  moou  near  the  third  qiiiirter.     From  a  photOLjruph  hy  I'rofecsor  Henry 

Draper. 

the  earlier  astronomers  all  faded  away,  leaving  our  satellite  as 
lifeless  as  an  arid  rock. 

As  the  moon  is  now  seen  and  mapped,  the  difference  be- 
tween the  light  and  dark  portions  is  due  merely  to  a  differ- 
ence in  the  color  of  the  nuiterial,  nnich  of  which  seems  to  be 


314  THE  SOLAR  SYSTEM. 

darker  than  the  average  of  terrestrial  objects.  The  mountains 
consist,  for  the  most  part,  of  round  saucer-shaped  elevations, 
the  interior  being  flat,  with  small  conical  mounds  rising  here 
and  there.  Sometimes  there  is  a  single  mound  in  the  centre. 
It  is  very  curious  that  the  figures  of  these  inequalities  in  the 
lunar  surface  can  be  closely  imitated  by  throwing  pebbles 
upon  the  surface  of  some  smooth  plastic  mass,  as  nmd  or 
mortar.  They  may  be  well  seen  during  an  eclipse  of  the  sun, 
when  the  contrast  between  the  smoothness  of  the  sun's  limb 
and  the  roughness  of  that  of  the  moon  cannot  escape  notice. 
Their  appearance  is  most  striking  when  the  eclipse  is  annular 
or  total.  In  the  latter  case,  as  the  last  streak  of  sunlight  is 
disappearing,  it  is  broken  up  into  a  number  of  points,  which 
have  been  known  as  "  Baily's  beads,"  from  the  observer  who 
first  described  them,  and  which  are  caused  by  the  sun  shining 
through  the  depressions  between  the  lunar  mountains. 

To  give  the  reader  an  idea  what  the  formation  of  the  lunar 
surface  is,  we  present  a  view  of  the  spot  or  crater  "  Coper- 
nicus," by  Secchi,  taken  from  the  "  Memoirs  of  the  Royal  As- 
tronomical Society,"  vol.  xxxii.  The  diameter  of  the  central 
portion,  so  much  like  a  fort,  is  about  45  or  50  miles. 

Among  the  most  curious  and  inexplicable  features  of  the 
moon's  surface  are  the  lone  narrow  streaks  of  white  material 
wliicli  radiate  from  certain  points,  especially  from  the  great 
crater  Tycho.  Some  of  these  can  be  traced  more  than  a 
thousand  miles.  The  only  way  in  which  their  formation  has 
been  accounted  for  is  by  supposing  that  in  some  former  age 
immense  fissures  were  formed  in  the  lunar  surface  which  were 
subsequently  filled  by  an  eruption  of  this  white  matter  which 
forms  the  streaks. 

Has  the  Moon  an  Atmosphere?  —  This  question  may  be  an- 
swered by  saying  that  no  evidence  of  a  lunar  atmosphere 
entitled  to  any  weight  lias  ever  been  gathered,  and  that  if 
there  is  such  an  atmosphere,  it  is  certainly  not  ^^^  part  the 
density  of  the  earth's  atmosphere.  The  most  delicate  known 
test  of  an  atmosphere  is  afforded  by  the  behavior  of  a  star 
when  in  apparent  contact  with  the  limb  of  the  moon.     In  this 


THE  MOON. 


315 


Fig.  si.— Lunar  crater  "Copernicus,"  al'ier  Secclii. 

position  llie  rays  of  light  coming  from  the  star  would  pass 
through  the  lunar  atmosphere,  and  be  refracted  by  twice  the 
horizontal  refraction  of  that  atmosphere.  The  star  would 
then  be  apparently  thrown  out  of  its  true  position  in  the  di- 
rection from  the  moon's  centre  by  the  amount  of  this  double 
refraction.  But  observations  of  stars  in  this  position,  at  the 
moment  v^hen  the  limb  of  the  moon  passes  over  them,  have 
never  indicated  the  slightest  displacement.  It  is  certain  that, 
had  the  displacement  been  decidedly  in  excess  of  half  a  sec- 
ond, it  would  have  been  detected ;  therefore,  the  double  hori- 
zontal refraction  of  the  lunar  atmosphere,  if  any  exist,  must 
be  as  small  as  half  a  second.*  The  corresponding  refraction 
of  the  earth's  atmosphere  is  4000  seconds.     Therefore,  the  re- 


*  A  similar  test  is  afforded  by  the  occultation  of  a  planet,  especially  Saturn  or 
Venus,  the  limb  of  which  would  he  a  little  flattened  as  it  touclied  tlie  moon.  The 
writer  looked  very  carefully  for  this  appearance  during  an  unusually  favorable  oc- 
cultation of  Suturn  which  occurred  on  Aug.  Gth,  1870,  without  seeing  a  trace  of  it. 


316  THE  SOL  AM  SYSTEM. 

fractive  power  of  the  lunar  atmosphere  cannot  be  much  in  ex- 
cess of  -g^oVo  that  of  the  earth's,  and  certainly  falls  below  -juVu- 

Without  an  atmosphere  no  water  or  other  volatile  fluid  can 
exist  on  the  moon,  because  it  would  gradually  evapoi'ate  and 
form  an  atmosphere  of  its  own  vapor.  The  evaporation  Mould 
not  cease  till  the  pressure  of  the  vapor  became  equal  to  its 
elastic  force  at  the  mean  temperature  of  the  moon.  If  this 
temperature  were  as  low  as  the  fi-eezin^-point,  the  pi-essure  of 
an  atmosphere  of  water  vapor  would  be  yjl^  that  of  our  at- 
mosphere. So  dense  an  envelope  could  not  fail  of  detection 
with  our  present  means  of  observation. 

The  question  whether  any  change  is  taking  place  on  the 
surface  of  the  moon  is  one  of  interest.  Hitheito,  the  pre- 
l>onderance  of  evidence  has  been  against  the  idea  of  any 
change.  It  is  true  that  a  few  years  ago  there  was  a  great 
discussion  in  the  astronomical  world  about  a  supposed  change 
in  the  aspect  of  the  spot  LinmBus,  which  was  found  not  to 
present  the  same  appearance  as  on  Beer  and  Miidler's  map. 
But  careful  scrutiny  showed  that,  owing  to  some  peculiarity 
of  its  surface,  this  spot  varied  its  aspect  accoi'ding  to  the 
manner  in  which  it  was  illuminated  by  the  sun,  and  these 
variations  appear  to  be  sufficient  to  account  for  the  supposed 
change.  To  whatever  geological  convulsions  the  moon  may 
have  been  subjected  in  ages  past,  it  seems  as  if  she  had  now 
reached  a  state  in  which  no  further  change  was  to  take  place, 
unless  by  the  action  of  some  new  cause.  This  M'ill  not  seem 
surprising  if  we  reflect  what  an  important  part  the  atmosphere 
plays  in  the  changes  which  are  going  on  on  the  surface  of  the 
earth.  The  growth  of  forests,  the  formation  of  deltas,  tlic 
washing-away  of  mountains,  the  disintegration  and  blacken- 
ing of  rocks,  and  the  decay  of  buildings,  are  all  due  to  the 
action  of  air  and  water,  the  latter  acting  i.\  the  form  of  rain. 
Changes  of  temperature  powerfully  re-enlorce  the  action  of 
these  causes,  but  are  not  of  themselves  sufficient  to  produce 
any  effect.  Now,  on  the  moon,  there  being  neither  air,  wa- 
ter, rain,  frost,  nor  organic  matter,  the  causes  of  disintegra- 
tion and  decay  are  all  absent.     A  marble  building  erected 


THE  MOON.  317 

upon  the  surface  of  tlie  moon  would  remain  century  after 
century  just  as  it  was  left.  It  is  true  that  there  miglit  be 
bodies  so  friable  that' the  expansious  and  contractions  due  to 
the  great  changes  of  temperature  to  which  the  surface  of  the 
moon  is  exposed  would  cause  them  to  crumble.  But  whatev- 
er crumbling  might  thus  be  caused  would  soon  be  done  with, 
and  then  no  further  change  would  occur. 

Light  and  Heat  of  the  Moon. — That  the  sun  is  many  times 
brighter  than  the  moon  is  evident  to  the  eye;  but  no  one 
judging  by  the  unaided  eye  would  suppose  the  disparity  to  be 
so  great  as  it  really  is.  It  is  found  by  actual  trial  that  the 
light  of  the  sun  must  be  diminislied  several  hundred  thousand 
times  before  it  becomes  as  faint  as  the  full  moon.  The  results 
of  various  experiments  range  between  300,000  and  800,000. 
Professor  G.  B.  Bond,  of  Cambridge,  found  the  ratio  to  be 
470,000.  The  most  careful  determination  yet  made  is  ])y 
Zcillner,  who  finds  the  sun  to  give  619,000  times  as  much 
light  as  the  full  moon.  This  result  is  probably  quite  near 
the  truth. 

The  moon  does  not  shine  by  sunlight  alone.  Whenever 
the  narrow  crescent  of  the  new  moon  is  seen  through  a  clear 
atmosphere,  her  whole  surface  may  be  plainly  seen  faintly  il- 
luminated. This  appearance  is  known  as  "  the  old  moon  in 
the  new  moon's  arms."  The  faint  light  thus  shed  upon  tiie 
dark  parts  of  the  moon  is  reflected  from  the  earth.  An  ob- 
server on  the  moon  would  see  the  earth  in  his  sky  as  a  large 
moon,  much  larger  than  the  moon  is  seen  by  us.  When  it  is 
new  moon  with  us,  it  would  he  full  earth,  if  we  may  be  allowed 
the  term,  to  an  observer  on  this  side  of  the  moon.  Hence, 
under  tliose  circumstances,  most  of  the  lunar  hemisphere  hid- 
den by  the  sun  is  illuminated  by  earth-light,  or  by  sunlight  re- 
flected by  the  earth,  and  is  thus  rendered  visible.  The  case 
is  the  same  as  if  an  observer  on  the  moon  sliould  see  the  dark 
hemisj)here  of  the  earth  by  the  light  of  the  full  moon. 

As  the  moon  reflects  the  light  of  the  sun,  so  also  must  she 
reflect  his  heat.  Besides,  she  tnust  radiate  off  whatever  heat 
she  absorbs  from  the  sun.     Hence,  we  must  receive  some    oat 


318  THE  SOLAR  SYSTEM. 

from  the  moon,  though  calculation  will  show  the  quantity  to 
be  so  small  as  to  defy  detection  with  the  most  delicate  ther- 
mometer, the  average  quantity  being  only  -s-ffTrViTTr  part  of  that 
received  from  the  sun.  As  the  direct  rays  of  the  sun  will  not 
raise  the  black-bulb  thermometer  more  than  50  or  60  degrees 
above  the  temperature  of  the  air,  those  of  the  moon  cannot 
raise  it  more  than  s^mr  of  a  degree.  J3y  concentrating  tlie 
rays  in  the  focus  of  a  telescope  of  large  aperture  and  compar- 
atively short  focal  length,  the  temperature  might  be  increased 
a  hundred  times  or  more  ;  but  even  then  we  should  only  have 
an  increase  of  -^V  of  a  degree.  Even  this  increase  might  be 
unattainable,  for  tlie  reason  that  tlie  heat  radiated  by  the 
moon  would  not  pass  through  glass.  It  is,  therefore,  only 
since  the  discovery  of  thermo  electricity  and  the  invention  of 
the  thermo-electric  pile  that  the  detection  of  the  heat  from 
the  moon  has  been  possible.  The  detection  is  facilitated  by 
using  a  reflecting  telescope  to  concentrate  the  lunar  rays, 
because  the  moon  is  not  hot  enough  to  radiate  such  heat  as 
will  penetrate  glass.  Lord  Rosse  and  M.  Marie  -  Davy,  of 
Paris,  have  thus  succeeded  in  measuring  thie  heat  emanating 
from  the  moon.  Tlie  former  sought  not  merely  to  determine 
the  total  amount  of  heat,  but  how  much  it  varied  from  one 
phase  of  the  moon  to  the  other,  and  what  portion  of  it  was 
the  reflected  heat  of  the  sun,  and  what  portion  was  radiated 
by  the  moon  herself,  as  if  she  were  a  hot  body.  lie  found 
that  from  new  to  full  moon,  and  thence  round  to  new  moon 
again,  the  quantity  of  heat  received  varied  in  the  same  way 
with  the  quantity  of  light ;  that  is,  there  was  most  at  full- 
moon,  and  scarcely  any  when  the  moon  was  a  thin  crescent. 
That  only  a  small  proportion  of  the  total  heat  emitted  was  the 
reflected  heat  of  the  sun,  was  shown  by  the  fact  that  while  86 
per  cent,  of  solar  heat  passes  through  glass,  only  12  per  cent, 
of  lunar  heat  does  so.  This  absorption  by  glass  is  well  known 
to  be  a  property  of  the  heat  radiated  by  a  body  which  is  not 
itself  at  a  high  temperature.  The  same  result  was  indicated 
in  another  way,  namely,  that  while  the  sun  is  found  by  Zoll- 
ner  to  give  618,000  times  as  much  light  as  the  moon,  it  only 


THE  MOON.  319 

gives  82,600  times  as  much  heat.  Thus  both  tlie  ratio  of  solar 
to  hiiiar  heat,  and  the  proportion  of  the  latter  which  is  ab- 
sorbed by  glass,  agree  in  indicating  that  about  six-sevenths  of 
the  heat  received  from  the  moon  is  radiated  by  the  latter, 
owing  to  the  temperature  of  her  surface  produced  by  the  ab- 
sorption of  the  sun's  rays. 

Lord  Rosse  was  thus  enabled  to  estimate  the  change  of 
temperature  of  the  moon's  surface  according  as  it  was  turned 
towards  or  from  the  sun,  and  found  it  to  be  more  than  500° 
Fahrenheit.  But  there  was  no  way  of  determining  the  tem- 
peratures themselves  with  exactness.  Probably  when  the  sun 
does  not  shine  the  temperature  is  two  or  three  hundred  de- 
grees below  zero,  and  therefore  below  any  ever  known  on  the 
earth;  while  under  the  vertical  sun  it  is  as  much  above  zero, 
and  therefore  hotter  than  boiling  water. 

Effect  of  the  Afoon  on  the  Eartli, — We  have  already  explained, 
in  treating  of  gravitation,  how  the  attraction  of  the  moon 
causes  tides  in  the  ocean.  This  is  one  of  the  best-known  ef- 
fects of  lunar  attra(;tion.  It  is  known  from  theory  that  a  sim- 
ilar tide  is  produced  in  the  air,  affecting  the  height  of  the  ba- 
rometer ;  but  it  is  go  minute  as  to  be  entirely  masked  by  the 
changes  constantly  going  on  in  the  atmospheric  pressure  from 
other  causes.  There  is  also  reason  to  believe  that  the  oc^ur- 
rencq  of  eartiiquakes  may  be  affected  by  the  attraction  of  the 
moon ;  but  this  is  a  subject  which  needs  further  investiga- 
tion before  we  can  pronounce  with  certainty  on  a  law  of  con- 
nection. 

Thus  far  there  is  no  evidence  that  the  moon  directly  affects 
the  earth  or  its  inhabitants  in  any  other  way  than  by  her  at- 
traction, which  is  so  minute  as  to  be  entirely  insensible  except 
in  the  ways  we  have  described.  A  striking  illustration  of  the 
fallibility  of  the  human  judgment  when  not  disciplined  by  sci- 
entific training  is  afforded  by  the  opinions  which  have  at  vari- 
ous times  obtained  cuneicy  respecting  a  supposed  influence 
of  the  moon  on  the  weather.  Xeither  in  the  reason  of  the 
case  nor  in  observations  do  we  find  any  real  support  for  such 
a  theory.    It  must,  however,  be  admitted  that  opinions  of  this 


320 


THE  SOLAR  SYSTEM. 


>' 


character  are  not  confined  to  the  uneducated.  In  scientific 
literature  several  papers  are  found  in  which  long  series  of  me- 
teorological observations  are  collated,  which  indicate  that  the 
mean  temperature  or  the  amount  of  rain  had  been  subject  to 
a  slight  variation  depending  on  the  age  of  the  moon.  But 
there  was  no  reason  to  believe  that  these  changes  arose  from 
any  other  cause  than  the  accidental  vicissitudes  to  which  the 
weather  is  at  all  times  subject.  There  is,  perhaps,  higher  au- 
thority for  the  opinion  that  the  rays  of  the  full  moon  clear 
away  clouds;  but  if  we  reflect  that  the  effect  of  the  sun  it- 
self in  this  respect  is  not  very  noticeable,  and  that  the  full 
moon  gives  only  -guxroo  of  the  heat  of  the  sun,  this  opinion 
will  appear  extremely  improbable. 

§  6.  The  Planet  Mars. 

The  fourth  planet  in  the  order  of  distance  fi'om  the  sun, 
and  the  next  one  outside  the  orbit  of  the  earth,  is  Mars.  Its 
mean  distance  from  the  sun  is  about  141  millions  of  miles. 
The  eccentricity  of  its  orbit  is  such  that  at  perihelion  it  is  only 
128  millions  of  miles  from  the  sun,  while  in  aphelion  it  is  154 
millions  distant.  It  is,  next  to  Mercury,  the  smallest  of  the 
primary  planets,  its  diameter  being  little  more  than  4000 
miles.  It  makes  one;  revolution  in  its  orbit  in  less  than  two 
years  (more  nearly  in  687  days,  or  43|  days  short  of  two  Ju- 
lian years).  If  the  period  were  exactly  two  years,  it  would 
make  one  revolution  while  the  earth  made  two,  and  the  oppo- 
sitions would  occur  at  intervals  of  two  years.  But,  goitig  a 
little  faster  than  this,  it  takes  the  earth,  on  the  average,  fifty 
days  over  the  two  yeare  to  catch  up  to  it.  The  times  of  oppo- 
sition are  ihown  in  the  followinir  table : 


1873 April  27th, 

1875 June  20th. 


1877 September  oth. 

187D November  12th. 


The  times  of  several  subsequent  oppositions  may  be  found 
with  suflicient  exactness  for  the  identification  of  the  planet  by 
adding  two  years  and  two  months  for  every  opposition,  except 
during  the   spring  months,  when   only  one  month  is  to   be 


THE  PLANET  MARS.  321 

added.  Oppositions  will  occur  in  January,  1882,  and  Febru- 
ary, 1884.  At  the  times  of  opposition  Mars  rises  when  the 
sun  sets,  and  may  be  seen  during  the  entire  nigh 

Aspect  of  Mars. — Mars  is  easily  recognized  with  the  naked 
eye  when  near  its  opposition  by  its  fiery-red  light.  It  is  much 
more  brilliant  at  some  oppositions  than  at  others,  but  always 
exceeds  an  ordinary  star  of  the  first  magnitude.  The  varia- 
tions of  its  brilliancy  arise  from  the  eccentricity  of  its  orbit, 
and  the  consequent  variations  of  its  distance  from  the  earth 
and  the  sun.  The  pei'helion  otMars  is  in  the  same  longitude 
in  which  the  earth  is  on  August  27th;  and  when  an  opposition 
occurs  near  that  date,  the  planet  is  only  35  millions  of  miles 
from  the  earth.  This  is  about  the  closest  approach  which  the 
two  planets  can  ever  make.  When  an  opposition  occurs  in 
February  or  March  the  planet  is  near  its  aphelion — 154  mill- 
ions of  miles  from  the  sun  and  62  millions  from  the  earth. 
The  result  of  these  variations  of  distance  is  that  Mars  is  more 
than  four  times  brighter  when  an  opposition  occurs  in  August 
or  September  than  when  it  occuis  in  February  or  March.  The 
opposition  of  1877  (September  5th)  is  quite  remarkable  in  this 
respect,  as  it  occurs  only  nine  days  after  the  planet  has  passed 
its  perihelion.  At  that  time  Mars  will  form  a  conspicuous 
object  in  the  south-eastern  sk}'  during  tlie  early  evening. 

Mars  has  been  an  interesting  object  of  telescopic  research 
from  the  fact  that  it  is  the  planet  which  exhibits  the  greatest 
analogy  with  our  earth.  The  equatorial  regions,  even  with  a 
small  telescope,  can  be  distinctly  seen  to  be  divided  into  light 
and  dark  portions,  which  some  observers  suppose  to  be  conti- 
nents and  oceans.  Around  each  pole  is  a  region  of  brilliant 
white,  which  the  same  class  of  astronomers  suppose  to  be  due 
to  a  deposit  of  snow.  The  outlines  of  the  dark  and  light  por- 
tions are  sometimes  so  hard  to  trace  as  to  give  rise  to  the  sus- 
picion of  clouds  in  a  Martial  atmosphere.  At  the  same  time, 
a  single  look  at  Mars  through  a  large  telescope  would  convince 
most  observers  that  these  resemblances  to  our  earth  have  a 
very  small  foundation  in  observation,  the  evidence,  being  neg- 
ative rather  than  positive.     It  must  be  said  in  their  favor  that 

22 


322 


THE  SOLAR  SYSTEM. 


if  our  earth  were  viewed  at  the  distance  at  which  we  view 
Mars,  and  with  the  same  optical  power,  it  would  present  a 
similar  telescopic  aspect.     But  it  is  also  possible  that  if  the 

optical  power  of  our  tele- 
scopes were  so  increased 
that  we  could  see  Mars  as 
from  a  distance  of  a  thou- 
sand miles,  the  resemblances 
would  all  vanish  as  com- 
pletely as  they  did  in  the 
case  of  the  moon. 

So  many  drawings  of 
Mai's  in  various  positions 
have  been  made  by  the  nu- 
merous observers  who  have 
studied  it,  that  it  has  be- 
come possible  to  construct 
toleiably  accurate  maps  of 
the  surface  of  the  planet.  We  give  a  copy  of  one  of  these 
sets  of  maps  by  Kaiser,  the  late  Leyden  astronomer.  Kaiser 
does  not  pretend  to  call  the  different  regions  continents  and 
oceans,  but  merely  designates  them  as  light  and  dark  portions. 


Fio.  82 The  planet  Mnis  on  June  23d,  1S76,  nt  10 

hours  46  minutes,  ns  seen  by  Professor  Uolden 
with  the  great  Washington  telescope. 


k9  M    M   m    «   JMf  JW  JUW7JV  iff  M»  tM  M*  ti0  H§  IM  J*«  IM  / 


Fio.  83 Map  of  Mars,  after  Kaiser,  on  Mercator's  projection. 

Rotatioyi  of  Mars. — Mars  is  the  only  planet  besides  the  earth 
of  which  we  can  be  sure  that  the  time  of  axial  rotation  ad- 
mits of  being  determined  with  entire  precision.  Drawings  by 
Hooke,  two  centuries  ago,  exhibit  markings  which  can  still  be 
recognized,  and  from  a  comparison  of  them  with  recent  ones 
Mr.  Proctor  has  found  for  the  period  of  rotation  24  hours  37 


THE  SMALL  PLACETS. 


323 


minutes  22.73  seconds,  which  he  considers  correct  within  three 
or  four  liundredths  of  a  second.  The  equator  of  Mars  is  in- 
clined to  tlie  phme  of  its  orbit  about  27°, so  that  the  vicissitudes 
of  the  seasons  are  greater  on  Mars  than  on  the  earth  in  the  pro- 
portion of  27°  to  23^°.  Owing  to  this  great  obhquity,  we  can 
sometimes  see  one  pole  of  the  planet,  and  sometimes  the  other, 
from  the  earth.     When  in  longitude  350°,  that  is,  in  the  same 


Fio.  84.— Northern  hemisphere  of  Mars. 


Fia.  85,— Southern  hemisphere  of  Mars. 


direction  from  the  sun  in  which  the  earth  is  situated  on  Sep- 
tember 10th,  the  south  pole  of  the  planet  is  inclined  towards 
the  sun ;  and  if  the  planet  is  then  in  opposition,  it  will  be  in- 
clined towards  the  earth  also,  so  that  we  can  see  the  region  of 
the  planet  to  a  distance  of  27°  beyond  the  pole.  At  an  op- 
position in  March  the  north  pole  of  the  planet  is  inclined  tow- 
ards the  sun,  and  towards  the  earth  also.  We  have  just  seen 
that  Mars  is  much  farther  at  the  latter  oppositions  than  at  the 
former,  so  that  we  can  get  much  better  views  of  the  south  polo 
of  the  planet  than  of  the  north  pole. 

§  7.  The  Small  Planets. 

It  was  impossible  to  study  the  solar  system,  as  it  was  known 
to  modern  astronomy  before  the  beginning  of  the  present  cent- 
ury, without  being  struck  by  the  great  gap  which  existed  be- 
tween Mars  and  Jupiter.  Except  this  gap,  all  the  planets  then 
known  succeeded  each  other  according  to  a  tolerably  regular 


324  THE  SOLAR  SYSTEM. 

law,  and  by  interpolating  a  single  planet  at  nearly  double  the 
distance  of  Mars  the  order  of  distances  would  be  complete. 
The  idea  that  an  unknown  planet  might  really  exist  in  this 
region  was  entertained  from  the  time  of  Kepler.  So  sure 
were  sotne  astronomers  of  this  that,  in  1800,  an  association  of 
twenty-four  observers  was  formed,  having  for  its  object  a  sys- 
tematic search  for  the  planet.  Tlie  zodiac  was  divided  into 
twenty-four  parts,  one  of  wliich  was  to  be  searched  through 
by  each  observer.  But  by  one  of  those  curious  coincidences 
which  have  so  frequently  occurred  in  the  history  of  science, 
the  planet  was  accidentally  discovered  by  an  outside  astrono- 
mer before  the  society  could  get  fairly  to  work.  On  January 
1st,  1801,  Piazzi,  of  Palermo,  found  a  star  in  the  constellation 
Taurus  which  did  not  belong  there,  and  on  observing  it  the 
night  after,  he  found  that  it  had  changed  its  position  among 
the  surrounding  stars,  and  must,  therefore,  be  a  planet.  He 
followed  it  for  a  period  of  about  six  weeks,  after  which  it  was 
lost  in  the  rays  of  the  sun  without  any  one  else  seeing  it. 
When  it  was  time  to  emerge  again  in  the  following  autumn, 
its  rediscovery  became  a  difficult  problem.  But  the  skill  of  the 
great  mathematician  Gauss  came  to  the  rescue  with  a  method 
by  which  the  orbit  of  any  planetary  body  could  be  complete- 
ly and  easily  determined  from  three  or  four  observations.  He 
was  thus  able  to  tell  observers  where  their  telescopes  must  be 
pointed  to  rediscover  the  planet,  and  it  was  found  without  dif- 
ficulty before  the  end  of  the  year.  Piazzi  gave  it  the  name 
Ceres.  The  orbit  found  by  Gauss  showed  it  to  revolve  between 
Mars  and  Jupiter  at  a  little  less  than  double  the  distance  of 
the  former,  and  therefore  to  be  the  long -thought -of  planet. 
But  the  discovery  had  a  sequel  which  no  one  anticipated,  and 
of  which  we  have  not  yet  seen  the  end.  In  March,  1802,  Gi- 
bers discovered  a  second  planet,  which  was  also  found  to  be 
revolving  between  Mars  and  Jupiter,  and  to  which  he  gave 
the  name  Pallas.  The  most  extraordinary  feature  of  its  orbit 
was  its  great  inclination,  which  exceeded  34°.  Gibers  there- 
upon suggested  his  celebrated  hypothesis  that  the  two  bodies 
might  be  fragments  of  a  single  planet  which  had  been  shat- 


THE  SMALL  PLANETS.  325 

tered  by  some  explosion.  If  such  were  the  case,  the  orbits  of 
all  the  fragments  would  at  first  intersect  each  other  at  the 
point  where  the  explosion  occurred.  lie  therefore  thought  it 
likely  that  other  fragments  would  be  found,  especially  if  a 
search  were  kept  up  near  tbe  point  of  intersection  of  the  orbits 
of  Ceres  and  Pallas.  Acting  on  this  idea,  Harding,  of  Lilien- 
thal,  found  a  third  planet  in  1804,  while  Gibers  found  a 
fourth  one  in  1S07.  These  were  called  Jimo  and  Vesta.  The 
former  came  quite  near  to  Olbers's  theory  that  the  orbits 
should  all  pass  near  the  same  point,  but  the  latter  did  not. 
Gibers  continued  a  search  for  additional  planets  of  this  group 
for  a  number  of  years,  but  at  length  gave  it  up,  and  died 
without  the  knowledge  of  any  but  these  four. 

In  December,  1845,  thirty-eight  years  after  the  discovery  of 
Vesta,  Hencke,  of  Driesen,  being  engaged  in  the  preparation 
of  star-charts,  found  a  fifth  planet  of  the  group,  and  thus  re- 
commen.ced  a  series  of  discoveries  which  have  continued  till 
the  present  time.  No  less  than  three  were  discovered  in  1847, 
and  at  least  one  has  been  found  every  year  since.  To  show 
the  rate  at  which  discovery  has  gone  on,  we  divide  the  time 
since  1845  into  periods  of  five  years  each,  and  give  the  num- 
ber found  during  each  period  : 


In  1840-50 8  were  discovered. 

"    1851-55 24     "  " 

"    185G-G0 25     "  " 


In  1801-65 23  were  discovered. 

"  18G6-70 27     "  " 

"  1871-75 45     "  " 


In  1876,  12  were  discovered,  and  three  additional  ones  have  been  found  durinj; 
tiie  first  five  months  of  1877,  making  a  total  of  172  known  at  the  present  time 
(May,  1877). 

It  will  be  seen  tliat  the  rate  of  discovery  has  been  pretty 
steadily  increasing  during  thirty  years.  This  is  not  because 
the  number  of  those  visible,  but  not  yet  found,  is  so  great  that 
it  is  as  easy  as  ever  to  find  one,  but  because  they  are  now 
sought  after  with  more  skill  and  more  system  than  formerly.* 

*  In  illustration  of  this  tlie  writer  has  heen  informed  by  Professor  Peters  that 
in  searching  for  tliese  bodies  he  falls  upon  several  already  known  for  every  new  one 
that  he  finds.  Consequently,  were  they  all  lost,  ho  alone  could  now  rediscover 
them  at  a  more  rapid  rate  than  they  actually  have  been  discovered  by  the  efforts 
of  all  the  observers  engaged  in  the  search. 


320  TUE  SOLAR  SYSTEM. 

Of  tliose  discovered  diiruig  tlic  last  ten  years,  nearly  half 
have  been  found  by  two  American  observers,  I'rofessors  Pe- 
ters and  Watson.  American  discoveries  of  these  bodies  were 
connnenccd  by  Mr.  James  Ferguson,  who  discovered  Euphros- 
yne  at  Washington  on  Septcniber  1st,  1854. 

All  the  planets  of  this  group  are  remarkable  for  their  mi- 
nuteness. The  disks  are  all  so  small  as  to  defy  exact  meas- 
urement, presenting  the  appearance  of  mere  stars.  A  rough 
estimate  of  their  diameters  can,  however,  be  made  from  the 
amount  of  light  which  they  reflect;  and  although,  in  the  ab- 
sence of  exact  knowledge  of  their  reflecting  ])ower,  the  results 
of  this  method  are  not  very  (tertain,  they  are  the  best  we  can 
obtain.  It  is  tlnis  found  that  Ceres  and  Vesta  are  the  largest 
of  the  group,  their  diameters  lying  somewhere  between  200 
and  400  miles;  while,  if  we  omit  some  very  lately  discovered, 
the  smallest  are  Atalanta,  Maja,  and  Sappho,  of  which  the  di- 
ameters may  be  between  20  and  40  miles.  We  may  safely 
say  that  it  would  take  several  thousand  of  the  largest  of  these 
small  planets  to  make  one  as  large  as  the  earth. 

It  has  sometimes  been  said  that  some  of  these  bodies  are  of 
irregular  shape,  and  thus  favor  Oibers's  hypothesis  that  they 
are  fragments  of  an  exploded  planet.  But  this  opinion  has 
no  other  foundation  than  a  suspected  variability  of  their  light, 
which  may  be  an  illusion,  and  wliich,  if  it  exists,  might  result 
from  one  side  of  the  planet  being  darker  in  color  than  the 
other.  The  latter  supposition  is  not  at  all  improbable,  as  many 
of  the  satellites  are  known  to  be  variable  from  this  or  some 
analogous  cause.  As  the  supposed  irregularities  of  form  liave 
never  been  seen,  and  are  not  necessary  to  accomit  for  the  va- 
riations of  brilliancy,  there  is  no  sufficient  reason  for  believing 
in  their  existence. 

Gibers  s  Hypothesis.  —  The  question  whether  these  bodies 
could  ever  have  formed  a  simple  one  has  now  become  one  of 
cosmogony  rather  than  of  astronomy.  If  a  planet  were  shat- 
tered, the  orbit  of  each  fragment  would,  at  flrst,  pass  through 
the  point  at  which  the  explosion  occurred,  however  widely 
they  might  be  separated  through  the  rest  of  their  course.    But 


THE  SMALL  I'LANETS.  327 

owing  to  the  secular  cliangcs  produced  by  the  attractions  of 
the  other  phmets,  this  coincidence  would  not  continue.  The 
orbits  would  slowly  move  away,  and  after  the  lapse  of  a  few 
thousand  years  no  trace  of  a  common  intersection  would  be 
seen.  It  is,  therefore,  curious  that  Gibers  and  his  contempora- 
ries should  have  expected  to  find  such  a  region  of  intersection, 
as  it  implied  that  the  explosion  had  occurred  within  a  few 
thousand  years.  The  fact  that  the  required  conditions  were 
not  f  ultilled  was  no  argument  against  the  hypothesis,  because 
the  explosion  might  have  occurred  millions  of  years  ago,  and 
in  the  mean  time  the  perihelion  and  node  of  each  orbit 
would  have  made  many  entire  revolutions ;  so  that  the  orbits 
would  have  been  completely  mixed  up. 

Desirous  of  seeing  whether  the  orbits  passed  nearer  a  com- 
mon point  of  intersection  in  times  past  than  at  present,  Encke 
computed  their  secular  variations.  The  result  seemed  to  be 
adverse  to  Olbers's  hypothesis,  as  it  showed  that  the  orbits 
were  farther  from  having  a  common  point  in  ages  past  than 
at  present.  But  this  result  was  not  conclusive  either,  beca\ise 
he  only  determined  the  rates  at  which  the  orbits  are  now 
changing,  whereas,  as  previously  explained,  the  orbits  of  all 
the  planets  really  go  through  periodic  oscillations ;  and  it  is 
only  by  calculating  these  oscillations  that  their  positions  can 
be  determined  for  very  remote  epochs.  They  have  since 
been  determined  for  some  of  the  planets  in  question,  and  the 
result  seems  to  show  that  the  orbits  could  never  have  intersect- 
ed unless  some  of  them  liave,  in  the  mean  time,  been  altered 
by  the  attraction  of  the  small  planets  on  each  other.  Such  an 
action  is  not  impossible;  but  it  is  impossible  to  determine  it, 
owing  to  the  great  number  of  these  bodies,  and  our  ignorance 
of  their  masses.  We  can,  however,  say  that  if  the  explosion 
ever  did  occur,  an  immense  interval,  probably  millions  of 
years,  must  have  elapsed  in  the  mean  time.  A  different  ex- 
planation of  the  group  is  given  by  the  nebular  hypothesis,  of 
which  we  shall  hereafter  speak,  so  that  Olbers's  iiypothesis  is 
no  longer  considered  by  astronomers. 

The  planets  in  question  are  distinguished  from  the  others, 


328  THE  SOLAR  SYSTEM. 

not  only  by  their  small  size,  but  by  the  great  eccentricities 
and  inclinations  of  their  orbits.  If  we  except  Mercury,  none 
of  the  larger  planets  has  an  eccentricity  amounting  to  one- 
tenth  the  diameter  of  its  orbit,  nor  is  any  orbit  inclined  more 
than  two  or  three  degrees  to  the  ecliptic.  But  the  inclina- 
tions of  many  of  the  small  planets  exceed  ten  degrees,  and 
the  eccentricities  frequently  amount  to  a  fourth  of  the  radii 
of  their  orbits.  The  result  is  that  the  same  small  planet  is  at 
very  different  distances  from  the  sun  in  various  points  of  its 
orbit.  Add  to  this  the  fact  that  the  mean  distances  of  these 
bodies  from  the  sun  have  a  pretty  wide  range,  and  we  shall 
find  that  they  extend  through  a  quite  broad  zone.  The  inside 
edge  of  this  zone  seems  pretty  well  marked,  its  distance  being 
about  180  millions  of  miles  from  the  sun,  or  between  30  and 
40  millions  beyond  the  orbit  of  Mars.  On  the  outside,  it  ter- 
minates more  gradually,  but  nowhere  extends  within  50  mill- 
ions of  miles  of  the  orbit  of  Jupiter.  If  any  of  the  small 
planets  ever  ranged  outside  of  certairi  limits,  the  attraction  of 
Mars  or  Jupiter  was  so  great  as  to  completely  derange  their 
orbits,  so  that  we  have  a  physical  law  which  sets  a  limit  to  the 
zone;  but  whether  the  limit  thus  set  would  coincide  with  the 
actual  limit  we  cannot  at  present  say. 

There  are  also  within  the  limits  of  the  group  certain  posi- 
tions, in  whi.  h,  if  the  orbits  were  placed,  they  would  be  greatly 
changed  by  the  action  of  Jupiter.  These  positions  are  those 
in  which  the  time  of  revolution  would  be  some  simple  exact 
fraction  of  that  of  Jupiter,  as  |^,  |-,  |,  y,  etc.  Professor  Daniel 
Kirkwood  has  pointed  out  the  curious  fact  that  tliere  are  gaps 
in  the  series  of  small  planets  corresponding  to  these  periodic 
times.  Whether  these  gaps  are  really  due  to  the  relations  of 
the  periodic  times,  or  are  simply  the  result  of  chance,  cannot 
yet  be  settled.  The  fact  that  quite  a  number  of  the  small 
planets  have  a  period  very  nearly  three-eighths  that  of  Jupiter, 
may  lead  us  to  wait  for  further  evidence  before  concluding 
that  we  have  to  deal  with  a  real  law  of  nature  in  the  cases 
pointed  out  by  Professor  Kirkwood. 

Number  and  Total  Mass  of  the  Small  Planets. — At  present  it 


THE  SMALL  PLANETS.  329 

is  not  possible  to  set  any  certain  limits  to  the  probable  number 
of  the  small  planets.  Although  a  hundred  and  seventy-two 
are  now  known,  there  is  as  yet  no  sensible  diminution  in  the 
rate  at  which  they  are  being  discovered.  The  question  of 
tlieir  total  number  depends  very  largely  on  whether  there  is 
any  limit  to  their  miimteness.  If  there  is  no  such  limit,  then 
there  may  be  an  indefinite  number  of  them,  too  small  to  be 
found  with  the  telescopes  now  engaged  in  searching  for  them; 
and  the  lai-ger  the  telescopes  engaged  in  the  search,  the  more 
will  be  found.  On  the  other  hand,  if  they  stop  at  a  certain 
limit — say  twenty  miles  in  diameter — we  may  say  with  con- 
sidei'able  coniidence  that  tlieir  total  number  is  also  limited, 
and  that  by  far  the  largest  part  of  them  will  be  discovered 
by  the  present  generation  of  astronomers. 

So  far  as  we  can  now  see,  the  preponderance  of  evidence  is 
on  the  side  of  the  number  and  magnitude  being  limited.  The 
indications  in  this  direction  are  that  the  newly  discovered  ones 
are  not  generally  the  smallest  objects  which  could  be  seen 
with  the  telescopes  which  have  made  the  discovery,  and  do 
not  seem,  on  the  average,  to  be  materially  smaller  than  those 
which  were  discovered  ten  years  ago.  It  is  not  likely  that  the 
number  of  this  avprage  magnitude  which  still  remain  undis- 
covered can  be  very  great,  and  new  ones  will  probably  be 
found  to  grow  decidedly  rare  before  another  hundred  are  dis- 
covered. Then  it  will  be  necessary  to  employ  greater  optical 
power  in  the  F-earch.  If  this  results  in  finding  a  number  of 
new  ones  too  si;  all  to  be  found  with  the  former  telescopes,  we 
shall  have  to  regard  the  group  as  uidimited  in  number.  But 
if  no  such  new  ones  are  thus  found,  it  will  show  that  the  end 
has  been  nearly  reached. 

In  gravitational  astronomy,  the  question  of  the  total  mass 
of  the  small  planets  is  more  important  than  that  of  their  total 
number,  because  on  this  mass  depends  their  effect  in  altering 
the  motions  of  the  large  planets.  Any  individual  small  planet 
is  so  mitiute  that  its  attraction  on  the  other  ])lanets  is  entirely 
insensible.  But  it  is  not  impossible  that  the  whole  group 
might,  by  their  combined  action,  produce  a  secular  variation 


330  THE  SOLAR  SYSTEM. 

in  the  fonn  of  the  orbits  of  Mars  and  Jupiter  which,  in  the 
course  of  years,  will  be  cleai'ly  shown  by  the  observations. 
But,  although  accurate  observations  of  these  planets  have  been 
made  for  more  than  a  century,  no  such  effect  has  yet  been  no- 
ticed. The  sum  total  of  their  masses  must,  therefore,  be  much 
less  than  that  of  an  average  planet,  though  we  cannot  say  pre- 
cisely what  the  limit  is.  The  apparent  magnitude  of  those 
which  have  been  discovered  is  entirely  accordant  with  the 
opinion  that  the  mass  of  the  entire  group  is  so  small  that  it 
caimot  make  itself  felt  by  its  attraction  on  the  other  planets 
for  many  years  to  come.  In  fact,  if  their  diameters  be  esti- 
mated from  their  brightness,  in  the  maimer  already  indicated, 
we  shall  find  that  if  all  that  are  yet  known  were  made  into  a 
single  planet  the  diameter  would  be  less  than  400  nnles;  and 
if  a  thousand  more,  of  the  average  size  of  those  discovered 
since  1850  should  exist,  their  addition  to  the  consolidated 
planet  w^onld  not  increase  its  diameter  to  500  miles.  Such  a 
planet  w'ould  be  only  ^rro  of  the  bulk  of  the  earth,  and,  un- 
less we  supposed  it  to  possess  an  extraordinary  specific  gravity, 
could  not  much  exceed  ^oo  oi  the  mass  of  the  earth,  or  -^V  of 
the  mass  of  Mercury.  We  may  fairly  conclude  that  unless 
the  group  of  small  planets  actually  consists  of  tens  of  thou- 
sands of  minute  bodies,  of  which  only  a  few  of  the  brightest 
have  yet  been  discovered,  their  total  volume  and  mass  are  far 
less  than  those  of  any  one  of  the  major  planets. 

The  number  of  these  bodies  now  known  is  so  great  that  the 
mere  labor  of  keeping  the  run  of  their  motions,  so  that  they 
shall  not  be  lost,  is  out  of  proportion  to  the  value  of  its  results. 
It  is  mainly  tln-ough  tlie  assiduity  of  German  students  that 
most  of  them  are  kept  from  being  lost.  Should  many  more 
be  found,  it  may  be  necessary  to  adopt  the  suggestion  of  an 
eminent  German  astronomer,  and  let  such  of  them  as  seem 
unimportant  go  again,  and  pursue  their  orbit  undisturbed  by 
telescope  or  cuniputer. 


THE  PLANET  JUPITER. 


3,'U 


CHAPTER  lY. 

THE  OUTER  GROUP  OF  PLANETS. 

§  1.  The  Planet  Jupiter. 

Jupiter  is  the  "giant  planet "  of  our  system,  his  mass  larijc- 
ly  exceeding  that  of  all  the  other  planets  combined,  llis 
mean  diameter  is  about  85,000  miles;  but  owing  to  his  rapid 
rotation  on  his  axis,  his  equatorial  exceeds  his  polar  diameter 


Fio.  80.— Jupiter  as  seen  with  the  prreat  Washington  telescope,  March  21st,  187C,  15  hours 
33  miuutus  mean  time.    Drawn  by  Professor  Iluideu. 

by  5000  miles.  In  volume  he  exceeds  our  earth  about  1300 
times,  while  in  mass  he  exceeds  it  about  213  times.  His  spe- 
cific gravity  is,  therefore,  far  less  than  that  of  the  earth,  and 
even  less  than  that  of  water.  His  moan  distance  from  the 
sim  is  480  millions  of  miles,  but,  owing  to  the  eccentricity  of 
his  orbit,  his  actual  distance  ranges  between  457  and  503  mill- 
ions. His  time  of  revolution  is  fifty  days  less  than  twelve 
years. 


332  THE  SOLAR  SYSTEM. 

Jnpiter  is  easily  recognized  by  his  brilliant  white  light,  with 
which  he  outshines  every  other  planet  excejjt  Venus.  To  fa- 
cilitate his  recognition,  we  give  the  dates  of  opposition  dur- 
ing a  few  years. 


1877 .Tune  l!)th, 

1878 July  2rjtli. 


187!) August  31st. 

1880 October  7th. 


Durhig  the  four  yeare  following  ISSO  he  will  be  in  opposition, 
on  the  average,  about  a  month  and  seven  days  later  each  year ; 
namely,  in  the  middle  of  November,  1881 ;  towards  the  latter 
part  of  December,  1882,  and  so  on.  A  month  or  two  before 
opposition  he  can  be  seen  rising  late  in  the  evening,  while 
during  the  three  months  following  opposition  he  will  always 
be  seen  in  the  early  evening  somewhere  between  south-east 
and  south-west. 

IVie  Surface  of  Jupiter. — Except  the  sun  and  moon,  there  is 
no  object  of  our  system  which  has  during  the  last  few  years 
been  the  subject  of  more  careful  examination  than  this  planet. 
Unlike  Mars,  there  are  no  really  permanent  markings  on  his 
surface,  and  a  map  of  Jupiter  is  therefore  imjjossible.  But 
this  surface  always  presents  a  very  diversified  a})[)earance. 
The  earlier  telescopic  observers  described  light  and  dark  belts 
as  extending  across  it.  Until  a  quite  recent  period,  it  has 
been  customary  to  describe  these  belts  as  two  in  number,  one 
north  of  the  equator,  and  the  other  south  of  it.  Commonly, 
they  are  seen  as  dark  bands  on  the  bright  disk  of  the  planet ; 
but  it  is  curious  that  Huyghens  represents  them  as  brighter 
than  the  rest  of  the  surface.  As  telescopic  power  was  in- 
creased, it  was  seen  that  these  so-called  bands  were  of  a  far 
more  complex  structure  than  had  been  supposed,  and  consisted 
of  great  numbers  of  stratified,  cloud-like  appearances  of  the 
most  variegated  forms.  These  forms  change  so  rapidly  that 
the  face  of  the  planet  hardly  ever  presents  the  same  appear- 
ance oil  two  successive  nights.  They  are  most  strongly 
marked  at  some  distance  on  each  side  of  the  Jovian  equator, 
and  thus  give  rise  to  the  appearaiute  of  two  belts  when  a  very 
small  or  imperfect  telescope  is  used. 


THE  PLANET  JVl'ITEE.  333 

Both  the  outlines  of  these  belts  and  the  color  of  some  parts 
of  the  planet,  seem  subject  to  considerable  changes.  The 
equatorial  regions,  and  indeed  the  spaces  between  the  belts 
generally,  are  often  of  a  rosy  tinge.  This  coloring  is  some- 
times so  strongly  marked  as  to  be  evident  to  the  most  super- 
ficial observer,  while  at  other  times  hardly  a  trace  of  it  can  be 
seen. 

Spots  which  are  much  more  permanent  Ihan  the  ordinary 
markings  on  the  belt  are  sometimes  visible.  By  watching 
these  spots  from  day  to  day,  and  measuritig  their  distance 
from  the  apparent  disk,  the  time  of  rotation  of  Jupiter  on  his 
axis  has  been  determined.  Commonly  the  spots  are  dark ; 
but  on  some  rather  rare  occasions  the  planet  is  seen  with  a 
number  of  small,  round,  bright  spots  like  satellites.  Of  these 
bright  spots  no  explanation  has  been  given. 


Fi«.  87.— View  of  Jupiter,  as  seen  in  Lord  Kosse's  great  telescope  on  Febrnary  27th, 

1S61,  at  12  hours  30  minutes. 

From  tliC  changeability  of  the  belts,  and  indeed  of  neai'ly  all 
the  visible  features  on  the  suivface  of  Jupiter,  it  is  clear  that 
what  we  see  on  that  planet  is  not  the  sm-face  of  a  solid  nu- 
cleus, but  vaporous  or  cloud-like  formations  which  cover  the 
entire  surface  and  extend  to  a  great  depth  below.  To  all  ap- 
pearance, the  planet  is  covered  with  a  deep  and  dense  atmbs- 


334  THE  SOLAR  SYSTEM. 

pliere,  through  whicli  light  cannot  penetrate  on  account  of 
thick  masses  of  clouds  and  vapor.  In  the  arrangements  of 
these  clouds  in  streaks  parallel  to  the  equator,  and  in  the 
change  of  their  forms  with  the  latitude,  there  may  he  some- 
thing analogous  to  the  zones  of  clouds  and  rain  on  the  earth. 
But  of  late  years  it  has  been  noticed  tliat  the  physical  consti- 
tution of  Jupiter  sof'ns  to  offer  more  analogies  to  that  of  the 
sun  than  to  that  ot  tiie  earth.  Like  the  sun,  he  is  brighter  in 
the  centre  than  near  the  edges.  This  is  shown  in  the  most 
striking  manner  in  the  transits  of  his  satellites  over  his  disk. 
When  the  satellite  first  enters  on  the  disk,  it  commonly  seems 
like  a  bright  spot  on  a  dark  background ;  but  as  it  approaches 
the  centre,  it  appears  like  a  dark  spot  on  the  bright  back- 
ground of  the  planet.  The  brightness  of  the  centre  is  prob- 
ably two  or  three  times  greater  than  that  of  the  limb.  This 
diminution  of  light  towards  the  edge  may  arise,  as  in  the  case 
of  the  sun,  from  the  light  near  the  edge  passing  through  a 
greater  depth  of  atmosphere,  and  thus  becoming  fainter  by 
absorption. 

A  still  more  remarkable  resemblance  to  the  sun  has  some- 
times been  suspected — nothing  less,  in  fact,  than  that  Jupiter 
shines  partly  by  his  own  light.  It  was  at  one  time  supposed 
that  he  actually  emitted  more  light  than  fell  upon  him  from 
the  sun ;  and  if  this  were  proved,  it  would  show  conclusive- 
ly that  he  was  self-luminous.  If  all  the  light  which  the  sun 
shed  upon  the  planet  were  equally  reflected  in  every  direction, 
we  might  speak  with  some  certainty  on  this  question ;  but  in 
the  actual  state  of  our  knowledge  we  cannot.  Zollner  has 
found  that  the  brightness  of  Jupiter  may  be  accounted  for  by 
supposing  him  to  reflect  62  per  cent,  of  the  sunlight  which  he 
receives.  But  if  this  is  his  average  reflecting  power,  the  re- 
flecting power  of  his  brighter  portions  must  be  much  greater; 
in  fact,  they  are  so  bright  that  they  must  shine  partly  by  their 
own  light,  unless  they  reflect  a  disproportionate  share  of  the 
sunlight  back  in-  the  direction  of  the  earth  and  sun.  Clouds 
would  not  be  likely  to  do  this.  On  the  other  hand,  if  we  as- 
sume that  the  planet  emits  any  great  amount  of  light,  we  are 


THE  PLANET  JUPITER.  335 

met  by  the  fact  that,  if  this  were  the  case,  the  satellites  would 
shine  by  this  light  when  they  were  in  the  shadow  of  the 
planet.  As  these  bodies  totally  disappear  in  this  position,  the 
quantity  of  light  emitted  by  Jupiter  must  be  quite  small.  On 
the  whole,  there  is  a  small  probability  that  the  brighter  spots 
of  this  planet  are  from  time  to  time  slightly  self-luminous. 

Again,  the  interior  of  Jupiter  seems  to  be  the  seat  of  an 
activity  so  enormous  that  we  can  attribute  it  only  to  a  very 
high  temperature,  like  that  of  the  sim.  This  is  shown  by  the 
rapid  movements  always  going  on  in  his  visible  surface,  which 
frequently  changes  its  aspect  in  a  few  hours.  Such  a  power- 
ful effect  could  hardly  be  produced  by  the  rays  of  the  sun, 
because,  owing  to  the  great  distance  of  the  planet,  he  receives 
only  between  one-twenty-fifth  and  one-thirtieth  of  the  light 
and  heat  which  we  do.  It  is  therefore  probable  that  Jupiter 
is  not  yet  covered  by  a  solid  crust,  as  our  earth  is,  but  that 
his  white-hot  interior,  whether  liquid  or  gaseous,  has  nothing 
to  cover  it  but  the  dense  vapors  to  which  that  heat  gives  rise. 
In  this  case  the  vapors  may  be  self-luminous  when  they  have 
freshly  arisen  from  the  interior,  and  may  rapidly  cool  off  after 
reaching  the  upper  limit  to  which  they  ascend. 

Rotation  of  Jupiter. — Owing  to  the  physical  condition  of  Ju- 
piter, no  precisely  determinate  time  of  rotation  can  be  assign- 
ed him,  as  in  the  case  of  Mars.  Without  a  solid  crnst  which 
we  can  see  from  time  to  time,  the  observed  times  of  rota- 
tion will  be  those  of  liquid  or  vaporous  formations,  which  may 
have  a  proper  motion  of  their  own.  A  spot  has,  however,  on 
some  occasions  been  observed  for  several  months,  and  it  Inis 
thus  been  pretty  certainly  determined  that  the  time  of  rota- 
tion is  about  9  hours  55^  minutes.  The  first  observation  of  a 
spot  of  this  kind  was  made  by  Cassini,  who  found  the  time  of 
rotation  to  be  9  hours  55  minutes  58  seconds.  No  further 
exact  observations  were  made  until  the  time  of  Schroter,  who 
observed  a  number  of  transient  spots  during  1785  and  1786. 
The  times  of  rotation  varied  from  9  hours  55  minutes  to  9 
hours  56  minutes,  from  which  he  concluded  that  heavy  storms 
raged  on  the  surface  of  the  planet,  and  gave  the  cloudy  masses 


336  THE  SOLAR  SYSTEM. 

which  formed  the  spots  a  motion  of  their  own.  In  Novem- 
ber, 1834,  a  remarkable  spot  was  observed  by  Miidler,  of  Dor- 
pat,  wliich  lasted  until  the  following  April,  from  which  the 
time  of  rotation  came  out  9  hours  55  minutes  30  seconds;  but 
the  observations  showed  that  the  spot  did  not  move  uniformly. 
Professor  Airy,  who  observed  the  same  spot  at  Cambridge, 
found  the  period  to  be  9  hours  55  minutes  21.3  seconds. 

Kecent  observations  and  researches  indicate  that  the  equa- 
torial regions  of  Jupiter  rotate  in  less  time,  and  with  more  ir- 
regularity, than  the  others,  thus  showing  still  another  analogy 
between  that  planet  and  the  sun.  Thus,  in  1871,  Dr.  Lohse, 
of  Bothkamp,  observed  a  spot  near  Jupiter's  equator,  which 
during  several  days  performed  its  revolution  in  a  period  of 
9  hours  51  minutes  47  seconds.  Other  equatorial  spots  had  a 
very  irregular  motion,  but  their  period  was  generally  less  than 
that  found  by  Miidler  and  Airy. 

§  2.  The  Satellites  of  Jupiter. 

One  of  the  earliest  telescopic  discoveries  by  Galileo  was 
that  Jupiter  was  accompanied  by  four  satellites,  which  re- 
volved round  him  as  a  centre,  thus  forming  a  miniature  copy 
of  the  solar  system.  As  in  the  case  of  spots  on  the  sun,  Gal- 
ileo's announcement  of  this  discovery  was  received  with  in- 
credulity by  those  philosophers  of  the  day  who  believed  that 
everything  in  nature  was  described  in  the  writings  of  Aris- 
totle. One  eminent  astronomer — Clavius  —  said  that  to  see 
the  satellites  one  must  liave  a  telescope  which  would  produce 
them ;  but  he  changed  his  mind  as  soon  as  he  saw  them  him- 
self. Another  philosopher,  more  prudent,  refused  to  put  his 
eye  to  the  telescope  lest  he  should  see  them  and  be  con- 
vinced, lie  died  shortly  afterwards.  "  I  hope,"  said  the  caus- 
tic Galileo,  "that  he  saw  them  while  on  his  way  to  lieaven." 

A  very  small  telescope,  or  even  a  good  opera-glass,  is  suf- 
ficient to  show  these  bodies.  Indeed,  very  strong  evidence  is 
on  record  that  they  have  been  seen  with  the  naked  e^'e.  That 
they  could  be  seen  by  any  good  eye,  if  the  planet  were  out  of 
the  way,  there  is  no  doubt,  the  difficulty  in  seeing  them  aris- 


THE  SATELLITES  OF  JUPITER.  337 

ing  from  the  glare  of  the  phinet  on  the  eye.  If  the  lenses  of 
the  eye  are  so  transparent  and  pure  that  there  is  no  such 
glare,  it  is  quite  possible  that  the  two  outer  satellites  might 
be  seen,  especially  if  they  should  happen  to  be  close  to- 
gether. 

According  to  the  best  determinations,  which  are,  however, 
by  no  means  certain,  the  diameters  of  the  satelUtes  of  Jupiter 
range  between  2200  and  3700  miles,  the  third  from  the  planet 
being  the  largest,  and  the  second  the  smallest.  The  volume  of 
the  smallest  is,  therefore,  very  near  that  of  our  moon. 

The  light  of  these  satellites  varies  to  an  extent  which  it 
is  difficult  to  account  for,  except  by  supposing  very  violent 
changes  constantly  going  on  on  their  surfaces.  It  has  some- 
times been  supposed  that  some  of  them,  like  our  moon,  always 
present  the  same  face  to  Jupiter,  and  that  the  changes  in  their 
brilliancy  are  due  to  differences  in  the  color  of  the  parts  of 
the  satellites  which  are  successively  turned  towards  us  during 
one  revolution  round  the  planet.  But  the  careful  measures 
of  their  light  made  by  Auwers,  of  Berlin,  and  Engelnumn,  of 
Leipsic,  show  that  this  hypothesis  does  not  account  for  the 
changes  of  brillianc}^  which  are  sometimes  sudden  in  a  sur- 
prising degree.  The  satellites  are  sn  distant  as  to  elude  tele- 
scopic examination  of  their  surfaces.  We  cannot,  therefore, 
hope  to  give  any  certain  explanation  of  these  changes. 

The  satellites  of  Jupiter  offer  problems  of  great  difficulty 
to  the  mathematician  who  attempts  to  calculate  the  effect  of 
their  mutual  attractions.  The  secular  variations  of  their  or- 
bits are  so  rapid  that  the  methods  applied  in  the  case  of  the 
planets  cannot  be  applied  here  without  material  alterations. 
The  most  curious  and  interestiuij  effect  of  their  mutual  at- 
traction  is  that  there  is  a  connection  between  the  motions  of 
the  three  inner  satellites  such  as  exists  nowhere  else  in  the 
solar  system.     The  connection  is  shown  by  these  two  laws : 

1.  That  the  mean  motion  of  the  first  satellite  added  to  twice  the 
mean  motion  of  the  third  is  exactly  equal  to  three  times  the  mean 
motion  of  the  second. 

2.  That  if  to  the  mean  longitude  of  the  first  satellite  tve  add 

23 


338  THE  SOLAR  SYSTEM. 

twice  the  mean  longitude  of  the  thinl,  and  .subtract  three  times  the 
mean  longitude  of  Ihc  second^  the  difference  is  alwai/s  180°. 

The  first  of  these  relations  is  shown  in  the  followinijc  table 
of  the  mean  daily  motions  of  the  satellites: 

Satellite     I.  in  one  day  moves 20;i°.48f»0 

"        II.        "         "  ior.3748 

"     in.      "       "        r.o\;5i77 

"      IV.        "         "  21°.r)71l 

Motion  of  Satellite  1 203^4H!)() 

Twice  that  of  Satellite  111 100°.<;:!r.-t 

Sum 8()4M244 

Three  times  motion  of  Satellite  II ;?()4M244 

It  was  first  found  from  observations  that  the  three  satellites 
moved  toijether  so  nearly  accordino;  to  this  law  that  no  certain 
deviation  could  be  detected.  But  it  was  not  known  whether 
this  was  a  mere  chance  coincidence,  or  an  actual  law  of  nat- 
ure, till  Laplace  showed  that,  if  they  moved  so  nearly  in  this 
way  as  observations  had  shown  them  to,  there  would  be  an  ex- 
tremely minute  force  arising  from  their  mutual  gravitation, 
sufficient  to  keep  them  in  this  relative  position  forever.  There 
is,  in  this  case,  some  analogy  to  the  rotation  of  the  moon, 
which,  being  once  started  presenting  the  same  face  to  the 
earth,  is  always  hold  in  that  position  by  a  minute  residual  of 
the  earth's  attraction. 

We  have  already  spoken  of  the  discovery  of  the  progressive 
motion  of  light  from  the  eclipses  of  these  satellites,  and  of 
the  uses  of  these  eclipses  for  the  rough  determination  of 
longitudes.  Both  the  eclipses,  and  the  transits  of  their  bodies 
over  the  face  of  Jupiter  afford  interesting  subjects  of  obser- 
vation with  a  telescope  of  sufficient  power,  say  four  inches  ap- 
erture or  upwards.  To  facilitate  such  observations  the  tinies 
of  these  phenomena  are  predicted  in  both  the  American  and 
British  Nautical  Almanacs. 

§  3.  Saturn  and  its  Si/stem,  Physical  Aspect^  Belts,  Rotation. 

Saturn  is  the  sixth  of  the  major  planets  in  the  order  of  dis- 
tance from  the  sun,  around  which  it  revolves  in  29^  years  at 


SATiltN  ASD  HIS  SYSTEM.  330 

a  mean  distance  of  about  880  millions  of  miles.  In  mass  and 
size  it  stands  next  to  Jui)iter.  To  show  the  disparity  in  the 
masses  of  the  i)ianets  we  may  refer  to  the  table  ali-eadv  •jjiven, 
showing  that  althoujjjh  Saturn  is  not  one -third  the  nuiss  of 
Jupiter,  it  lias  about  three  times  the  mass  of  the  six  planets, 
wliich  are  smaller  than  itself  put  toij^ether.  Its  surroundinirs 
are  such  as  to  make  it  tlie  most  nniij^niticent  object  in  the  solar 
system.     While  no  other  planet  is  known  to  have  more  than 


Fiu.  S8. — Viow  of  Satuiii  ami  his  riiitrs. 


fonr  satellites,  Saturn  has  no  less  than  ei2;bt.  It  is  also  snr- 
rounded  by  a  pair  of  rin2;s,  the  interior  diameter  of  which  is 
about  100,000  miles.  The  aspect  of  these  rings  is  subject  to 
great  variations,  for  reasons  which  will  soon  apjicar.  The 
great  distance  of  the  planet  renders  the  study  of  its  details 
diflicult  unless  the  highest  telescoi)i(!  power  is  ap]>lied.  The 
whole  combination  of  Saturn,  his  rings,  and  his  satellites  is 
often  called  the  Sdturnian  S//s(em. 

The  planet  Saturn  generally  shines  with  the  brilliancy  of  a 


340 


THE  SOLAR  SYSTEM. 


moderate  first-magnitude  star,  and  with  a  dingy,  reddish  light, 
as  if  seen  tlirough  a  smoky  atmosphere.  Its  apparent  bright- 
ness is,  howevei',  different  at  different  times:  dnring  tho  years 
1876-1879  it  is  fainter  than  the  average,  owing  to  its  ring  be- 
ing seen  wevvXy  edgewise.  From  1878  till  1885  it  will  con- 
stantly grow  brighter,  on  account  botli  of  tlie  opening  out  of 
the  ring  and  the  approach  of  the  planet  to  \ts  perihelion. 
Tiie  times  of  opposition  are  as  follow  : 


1877 September  9th. 

1878 September  22d. 


187!) October  .''.th. 

1880 October  18tb. 


In  subsequent  years  opposition  will  occur  about  thirteen  days 
later  every  year,  so  that  l)y  adding  this  amount  to  the  date  for 
each  year  the  oppositions  can  be  found  until  the  end  of  the 
century  without  an  error  of  more  than  a  few  days. 

The  physical  constitntion  of  Saturn  seems  to  bear  a  great 
resemblance  to  that  of  Jupiter ;  but,  being  twice  as  far  away, 
it  cannot  be  so  well  studied.  The  farther  an  object  is  from 
the  sun,  the  less  brightly  it  is  illuminated;  and  the  farther 
from  the  earth,  the  smaller  it  looks,  so  that  there  is  a  double 
difliculty  in  getting  the  finest  view^  of  the  more  distant  plan- 
ets. When  examined  under  favoi  XAe  circumstances,  the  sur- 
face of  Saturn  is  seen  to  be  diversified  with  very  faint  mark- 
ings ;  and  if  high  telescopic  powers  are  used,  two  or  more 
very  faint  streaks  or  belts  may  be  seen  parallel  to  its  equator, 
the  strongest  ones  lying  on,  or  very  near,  the  equator.  As  in 
the  ease  of  Jupiter,  these  belts  chan_,e  their  aspect  from  time 
to  tin:e,  but  they  are  so  faint  that  the  changes  cannot  be 
easily  followed.  It  is  therefore,  in  general,  difficult  t  sny 
with  certainty  whether  we  do  or  do  not  see  the  same  face  of 
Saturn  on  different  nights;  and,  consequently,  it  is  only  0!i 
extraordinary  occasions  that  the  time  of  rotaticn  can  be  de- 
termined. 

The  first  occasion  on  whicli  a  well-defined  spot  was  kno,vn 
to  remain  long  enough  on  Saturn  to  determine  the  period  of 
its  rotation  was  in  the  time  of  Sir  W.  Ilerschel,  who,  from 
observations  extending  over  several  weeks,  found  the  time  of 


THE  RINGS  OF  SATUliX.  341 

rotation  to  be  10  lionrs  16  minutes.*  No  further  opportu- 
nity for  determining  this  period  seems  to  have  offered  itself 
until  1876,  when  an  appearance  altogether  new  suddenly 
showed  itself  on  the  globe  of  this  planet.  On  the  evening  of 
December  7th,  i876,  Professor  Hall,  who  had  been  engaged 
in  measures  of  the  satellites  of  Saturn  with  the  great  Wash- 
ington telescope,  saw  a  brilliant  white  spot  near  the  equator 
of  the  planet.  It  seemed  as  if  an  immense  eruption  of  white- 
hot  matter  had  suddenly  burst  up  from  tlie  interior.  The 
spot  gradually  spread  itself  ouc  in  the  direction  which  would 
be  east  on  the  planet,  so  as  to  assume  the  form  of  a  long  light 
streak,  of  which  the  brightest  point  was  near  the  following 
end.  It  continued  visible  until  January,  when  it  became  faint 
and  ill-delined,  and  the  planet  was  lost  in  the  rays  of  the  sun. 
Immediately  upon  the  discovery  of  this  remarkable  phenom- 
enon, messages  were  sent  to  other  observers  in  various  parts  of 
the  country,  and  on  tlie  10th  it  was  seen  by  several  obserNei-s, 
who  noted  the  time  at  which  it  crossed  the  centre  of  the  disk 
in  consequence  of  the  rotation  of  the  planet.  From  all  the 
observations  of  this  kind.  Professor  Hall  found  the  period  of 
Saturn  to  be  10  hours  14  minutes,  taking  the  brightest  part 
of  the  streak,  which,  as  we  have  said,  was  near  one  end. 
Had  the  middle  of  the  streak  been  taken,  the  time  would  have 
been  less,  because  the  bright  matter  seemed  to  be  carried 
along  in  the  direction  of  the  planet's  rotation.  Attributing 
this  to  a  wind,  the  \elocity  of  the  latter  would  have  been  be- 
tween 50  and  100  miles  an  hour. 

§  4.  The  Rings  of  Saturn, 

Tiie  most  extraordinary  feature  of  Saturn  is  the  magnificent 
system  of  rings  by  which  he  is  surrounded.  To  the  early 
telescopists,  who  could  not  command  sufficient  o[)tical  power 
to  see  exactly  what  it  was,  this  feature  was  a  source  of  great 

*  It  is  very  curious  timt  nearly  nil  modern  writers  give  about  10  hours  29  min- 
utes as  the  time  of  rotation  of  Saturn  which  Ilerscliel  finally  deduced.  I  can 
tind  no  such  result  in  Ilerschel's  papers.  A  suspicious  coiiu'idcnce  i:*  tliat  this 
jiev'.od  ajj-c-C2  with  that  assigned  for  tlie  time  of  rotation  of  the  liug. 


342  THE  SOLAR  SYSTEM, 

perplexit}'  and  difference  of  opinion.  To  Galileo  it  made  the 
planet  appear  triform — a  large  globe  with  two  small  ones  af- 
fixed to  it,  one  on  each  side.  After  he  had  observed  it  for  a 
year  or  two,  he  was  greatly  perplexed  to  tind  that  the  append- 
ages had  entirely  disappeared,  leaving  Satnrn  a  single  round 
globe,  like  the  other  planets.  His  chagrin  was  heightened  by 
the  fear,  not  nnnatural  nnder  the  circumstances,  that  the  curi- 
ous form  he  had  before  seen  might  be  due  to  some  optical  il- 
lusion connected  with  his  telescope.  It  is  said  (I  do  not  know 
on  what  authoi'ity)  that  his  annoyance  at  the  supposed  decep- 
tion into  which  he  had  fallen  was  so  great  that  he  never  again 
looked  at  Saturn. 

A  very  few  years  sufficed  to  show  other  observers,  who  had 
connnand  of  more  powerful  telescopes,  that  the  singularity  of 
form  was  no  illusion,  but  that  it  varied  from  time  to  time. 
We  give  several  pictures  from  Iluyghens's  Syslema  /Satumiuin, 
showing  how  it  was  rejiresented  by  various  observers  during 
the  first  forty  years  of  the  telescope.  If  the  reader  will  com- 
pare these  with  the  picture  of  Saturn  and  his  rings  as  they 
actually  are,  he  will  see  how  near  many  of  the  observers  came 
to  a  representation  of  the  proper  a[)parent  form,  though  none 
divined  to  v'hat  sort  of  an  appendage  the  a})})earance  was 
due. 

The  man  who  at  last  solved  the  riddle  was  Iluyghens,  of 
whose  long  telescopes  we  have  already  si)()ken.  Examining 
Saturn  in  March  and  A])ril,  1(]55,  he  saw  that  instead  of  the 
appendages  presenting  the  appearance  of  curved  handles,  as 
in  previous  years,  a  long  narrow  arm  extended  straight  out  on 
each  side  of  the  ]>lanet.  The  spring  following,  this  arm  had 
disappeared,  and  the  planet  appeared  perfectly  round  as  Gal- 
ileo had  seen  it  in  1G12.  In  October,  1G55,  the  handles  had 
reapi)eared,  much  as  l>e  had  seen  them  a  year  and  a  half  be- 
fore. To  his  remarkably  acute  mathematical  and  mechnnical 
mind  this  mode  of  disappearance  of  the  handles  sufficed  to 
suggest  the  cause  which  led  to  their  ajiparent  form.  Waiting 
for  entire  confirmation  by  futrre  observations,  he  (iommunica- 
ted  his  theory  to  his  fellow-astronomers  in  the  following  com- 


THE  RINGS  OF  SATURN. 


343 


Fio.  89. —Specimens  of  dra\vini;s  of  Saturn  by  various  olipcrvers  before  the  rinirs  were 
recofrnized  as  sucli:  I.  Form  as  };iveu  by  Galileo  iii  1010;  II.  Drawing  by  Schciiier,  in 
1614,  "showing  ears  to  Saturn;"  III.  Drawing  by  Uicciolus,  in  1640  and  164;i;  IV., V., 
YI.,  and  VII.  are  by  Ilevelius,  and  show  the  changes  due  to  the  difTerent  angles  under 
which  the  rings  were  seen;  VIII.  and  IX.  are  by  Rireiolus,  between  164^  and  IfiBO, 
when  the  ring  was  seen  at  the  greatest  angle ;  X.  is  by  a  Jesuit  who  pasj^ed  under 
the  pseudonym  of  KiistarhiK.i  ih:  Diciniti;  XI.  is  by  Foutaua;  XII.  by  Gassendi  and 
Ulancanus,  and  XIII.  by  Hicciohis. 

bhiatioii  of  letters,  ]irinto(l  without  explanation  at  the  end  of  a 
little  pamphlet  on  his  discovery  of  the  satellite  of  Satnrn : 

aaaaaaa  ccccc  d  eecee  g  h  iiiiiii  llll  mm  nnnnnnnnn  oooo  pp  ij  rr  s  ttttt  uuiiuu, 

which,  properly  arrann;ed,  read — 

**  Annulo  vingitur,  teniti,  ph.  no,  nusquam  cohwrente,  ad  ecJipticam  inrJinato" 
(It  is  girdled  by  a  thin  plane  ring,  nowhere  touching,  inclined  to  the  ecliptic). 

This  description  is  remarkably  complete  and  accurate;  and 
enabled  Iluyghens  to  give  a  satisfaci.ory  explanation  of  th« 


344  THE  SOLAB  SYSTEM. 

various  phases  which  the  ring  had  assumed  as  seen  from  the 
earth.  Owing  to  the  extreme  thinness  and  flatness  of  the  ob- 
ject, it  was  completely  invisible  in  the  telescopes  of  that  time 
when  its  edge  was  presented  tow?.rds  the  observer  or  towards 
the  sun.  This  happens  twice  in  each  revolution  of  Saturn,  in 
much  the  same  way  that  the  earth's  equator  is  twice  directed 
towards  the  sun  in  tlie  course  of  the  year.  The  ring  is  in- 
clined to  the  plane  of  the  planet's  orbit  b}'  27°,  corresponding 
to  the  angle  of  23^°  betvveen  the  earth's  equator  and  the 
ecliptic.  The  general  aspect  from  the  earth  is  very  near  the 
same  as  from  the  sun.  As  the  planet  revolves  around  the 
sun,  the  axis  and  plane  of  the  ring  preserve  the  same  absolute 
direction  in  space,  just  as  the  axis  of  the  earth  and  the  plane 
of  the  equator  do. 

When  the  planet  is  in  one  part  of  its  orbit,  aa  observer  at 
the  sun  or  on  the  earth  will  see  the  upper  or  northern  side  of 
the  ring  at  an  inclination  of  27°.  Tliis  is  the  greatest  angle 
at  which  the  ring  can  ever  be  seen,  the  position  occurring 
when  the  planet  is  in  262°  of  longitude,  in  the  constellation 
Sagittarius.  When  the  planet  has  moved  through  a  quarter 
of  a  revolution,  the  edge  of  the  ring  is  turned  towards  '  e  sun, 
and,  owing  to  its  extreme  thiimess,  it  is  visible  on:/  in  the 
most  powerful  telescopes  as  an  exceedingly  tine  line  of  light, 
stretching  out  on  each  side  of  the  planet.  In  this  position  the 
planet  is  in  longitude  352°,  in  the  constellation  Pisces.  Wiien 
the  planet  has  moved  90°  farther,  an  observer  on  the  sun  or 
earth  again  sees  the  ring  at  an  angle  of  27°  ;  but  now  it  is  the 
lower  or  southern  side  which  is  visible.  The  planet  is  now  in 
longitude  82°,  between  the  constellations  Taurus  and  Gemini. 
When  it  has  moved  90°  farther,  to  longitude  172°,  in  the  con- 
stellation Leo,  the  edge  of  the  ring  is  again  turned  towards 
the  earth  and  sun. 

Tlius  there  are  a  pair  of  opposite  points  of  tlie  orbit  of  Sat- 
urn ill  which  the  rings  are  turned  edgewise  to  us,  and  another 
pair  half-way  between  the  first  in  which  the  ring  is  seen  at 
its  maximum  inclination  of  about  27°.  Since  the  planet  per- 
forms a  revolution  in  29^  years,  these  phases  occur  at  average 


THE  RINGS  OF  SATUIiX.  34:5 

intervals  of  about  seven  years  and  four  months.     The  follow- 
ing are  some  of  the  times  of  their  occurrence : 

1870.  The  planet  being  between  Scorpio  and  Sagittarius, 
the  ring  was  seen  open  to  its  greatest  breadth,  the  north  side 
being  visible.     Tlie  same  phase  recurs  at  the  end  of  1899. 

1878  (February  7th).  The  edge  of  the  ring  is  turned  tow- 
ards the  sun,  so  that  only  a  thin  line  of  light  will  be  visible. 
The  planet  is  then  between  Aquarius  and  Pisces. 

1885.  The  planet  being  in  Taurus  (the  Bull)  the  south  side 
of  the  rings  will  be  seen  at  the  greatest  elevation. 

1892.  The  edge  of  the  ring  is  again  turned  towards  the  sun, 
the  planet  being  in  Leo  (the  Lion). 

Owing  to  the  motion  of  the  earth,  the  times  when  the  edge 
of  the  ring  is  turned  towai'ds  it  do  not  accurately  correspond 
to  those  when  it  is  turned  towards  the  sun,  and  the  points  of 
Saturn's  orbit  in  which  this  may  occur  range  over  a  space  of 
several  degrees.  The  most  interesting  times  for  viewing  the 
rings  with  powerful  telescopes  are  on  those  rare  occasions 
when  the  sun  shines  on  one  side  of  the  ring,  wliile  the  dark 
side  is  directed  towards  the  earth.  On  these  occasions  the 
plane  of  the  ring,  if  extended  out  far  enough,  would  pass  be- 
tween the  sun  and  the  earth.  This  will  be  the  case  between 
February  9th  and  March  1st,  1878  ;  but,  unfoitunatel} ,  at  that 
time  the  earth  and  Saturn  are  on  opposite  sides  cf  the  sun,  so 
that  the  planet  is  nearly  lost  in  the  sun's  rays,  and  can  be  ob- 
served only  low  down  in  the  west  just  after  sunset.  In  1891 
the  position  of  Saturn  will  be  almost  equally  unfavorable  for 
the  observation  in  question,  as  it  can  be  made  only  in  the  early 
mornings  of  the  latter  part  of  October  of  that  year,  just  after 
Saturn  has  risen.  In  fact,  a  good  opportunity  will  not  occur 
till  1907.  In  northern  latitudes  the  finest  telescopic  views  of 
Saturn  and  his  ring  may  be  obtained  between  1881  and  1889, 
because  during  that  interval  Saturn  passes  his  perihelion,  and 
also  the  point  of  greatest  northern  declination,  while  the  ring 
is  opened  out  to  its  widest  extent.  In  fact,  these  three  most 
favorable  conditions  all  fall  nearly  together  during  the  years 
1881-'85.  "    . 


346  THE  SOLAR  SYSTEM. 

After  Iliiyghens,  tlie  next  step  forward  in  discoveries  on 
Saturn's  ring  was  made  by  an  English  observer,  named  Ball, 
otherwise  unknown  in  astronomy,  who  found  that  there  were 
i-eally  two  rings,  divided  by  a  narrow  dark  line.  The  breadth 
of  the  rings  is  very  unequal,  the  inner  ring  being  several  times 
broader  than  the  outer  one.  A  moderate  -  sized  telescope  is 
sufficient  to  show  this  division  near  the  extreme  points  of  the 
ring  if  the  atmosphere  is  steady ;  but  it  requires  both  a  large 
telescope  and  tine  seeing  to  trace  it  all  tlie  way  across  that 
part  of  the  ring  which  is  between  the  observer  and  the  ball  of 
the  planet.  Other  divisions,  especially  in  the  outer  ring,  have 
at  times  been  suspected  by  various  observei's,  but  if  they  real- 
ly existed,  they  must  have  been  only  temporary,  forming  and 
closing  up  again. 

In  December,  1850,  the  astronomical  world  was  surprised 
by  the  annoiuicement  that  Professor  Bond,  of  Cambridge,  had 
discovered  a  third  ring  to  Saturn.  It  lay  between  the  rings 
already  known  and  the  planet,  being  joined  to  the  inner  edge 
of  the  inner  ring.  It  had  the  appearance  of  a  ring  of  crape, 
being  so  dark  and  obscure  that  it  might  easily  have  been 
overlooked  in  smaller  telescopes.  It  was  seen  in  England  by 
Messrs.  Lassell  and  Dawes  before  it  was  formally  announced 
by  the  Bonds.  Something  of  the  kind  had  been  seen  by  Dr. 
Galle,  at  Berlin,  as  far  back  as  1838;  but  the  paper  on  the 
subject  by  Encke,  the  director  of  the  observatory,  did  not  de- 
scribe the  appearance  very  clearly.  Indeed,  on  examining  the 
descriptions  of  obseVvers  in  the  early  part  of  the  eighteenth 
century,  some  reason  is  found  for  suspecting  that  they  saw 
this  dusky  ring;  but  none  of  the  descriptions  are  sufficiently 
definite  to  establish  the  fact,  though  it  is  strange  if  an  object 
so  plain  as  this  ring  now  is  should  have  been  overlooked  by 
all  the  older  observers. 

The  question  whether  changes  of  various  sorts  are  going  on 
in  the  rings  of  Saturn  is  one  which  is  still  unsettled.  There 
is  some  reason  to  believe  that  the  supposed  additional  divis- 
ions noticed  in  the  rings  fi-oni  time  to  time  are  only  errors  of 
vision,  due  partly  to  the  shading  which  is  known  to  exist  on 


THE  RINGS  OF  SATUBX.  347 

various  parts  of  the  ring.  By  reference  to  the  diagram  of 
Saturn,  it  will  be  seen  that  the  outer  ring  has  a  shaded  line 
extending  around  it  about  two-thirds  of  the  way  from  its  in- 
ner to  its  outer  edge.  This  line,  however,  is  not  fine  and 
sharp,  like  the  known  division,  but  seems  to  shade  off  gradual- 
ly towards  each  edge.  As  observers  who  have  supposed  them- 
selves to  see  a  division  in  this  rins:  saw  it  where  this  shii<^"d 
line  is,  and  do  not  speak  of  the  latter  as  anything  distin*„t 
from  the  former,  there  is  reason  to  believe  that  they  mistook 
this  permanent  shading  for  a  new  division.  The  inner  ring  is 
brightest  near  its  outer  edge,  and  shades  off  gradually  towards 
its  inner  edge.  Here  the  dusky  ring  joins  itself  to  it,  and  ex- 
tends about  half-way  in  to  the  planet. 

As  seen  with  the  great  Washington  equatorial  in  the  au- 
tumn of  1874,  there  was  no  great  or  sudden  contrast  be- 
tween the  inner  or  dark  edge  of  the  bright  ring  and  the  out- 
er edge  of  the  dusky  ring.  There  was  some  suspicion  that 
the  one  shaded  into  the  other  by  insensible  gradations.  No 
one  could  for  a  moment  suppose,  as  some  observers  have,  that 
there  was  a  separation  between  these  two  rings.  All  these 
considerations  give  rise  to  the  question  whether  the  dusky 
ring  may  not  be  growing  at  the  expense  of  the  '  ler  bright 
ring. 

A  most  startling  theory  of  changes  in  the  rings  of  Saturn 
was  propounded  by  Struve,  in  1851.  This  was  nothing  less 
than  that  the  inner  edge  of  the  ring  was  gradually  approach- 
ing the  planet  in  consequence  of  the  whole  ring  spreading  in- 
wards, and  the  central  opening  thus  becoming  smaller.  The 
data  on  which  this  theory  was  founded  were  the  descriptions 
and  drawings  of  the  rings  by  the  astronomers  of  the  seven- 
teenth cen I  ury,  especially  Iluyghens,  and  the  measures  ex- 
ecuited  by  later  astronomers  up  to  the  time  at  which  Struve 
wrote.  The  rate  at  which  the  space  between  the  ring  and  the 
planet  was  diminishing  seemed  to  be  ahont  1".3  per  century. 
The  following  are  the  numbers  used  by  Struve,  which  arc  de- 
duced from  the  descriptions  by  the  ancient  observers,  and  the 
measures  by  the  modern  ones : 


348 


TUE  SOLAR  SYSTEM. 


Yenr. 

Hiivgliens 

1  fin? 

llinglKins  and  Cassiiii 

Hiadlev 

i(!i»r. 

171!) 

Ileiscliel 

1 7!)!) 

W.  Htnive 

IS'-'O 

Encke  and  Galle 

Otto  Stiuve 

1 8;]8 
1851 

i)i 

*tHnrt>  bt'twecri 

Ri 

!(,'  »li(i  Plnnet. 

fi.r, 

«>.o 

5.4 

5.12 

4.3G 

4.04 

3.  (J  7 

llrcndth  of 
Ulriir. 


4.r, 

5.1 

5.7 

5.98 

G.74 

7.0« 

7.43 


If  these  estimates  and  measures  were  certainly  accurate, 
they  would  place  the  fact  of  a  progressive  approach  of  the 
rings  to  the  ball  beyond  doubt,  an  approach  which,  if  it  con- 
tinued at  the  same  rate,  would  bring  the  inner  edge  of  the 
ring  into  contact  with  the  planet  about  the  year  2150.  But 
in  measuring  such  an  object  as  the  inner  edge  of  the  ring  of 
Saturn,  which,  as  we  have  just  said,  seems  to  fade  gradually 
into  the  obscure  ring,  different  observers  will  always  obtain 
different  results,  and  the  differences  among  the  four  observ- 
ers commencing  with  W.  Struve  are  no  greater  than  are  often 
seen  in  measuring  an  object  of  such  uncertain  outline.  Hence, 
considering  the  great  improbability  of  so  stupendous  a  cosmi- 
cal  change  going  on  with  so  much  rapidity,  Struve's  theory  has 
always  been  viewed  with  doubt  by  other  astronomers. 

At  the  same  time,  it  is  impossible  to  reconcile  the  descrip- 
tions by  the  early  observers  with  the  obvious  aspect  of  the 
ring  as  seen  now  without  supposing  some  change  of  the  kind. 
The  most  casual  observer  who  now  looks  at  Saturn  will  see 
that  the  breadth  of  the  two  bright  rings  together  is  at  least 
half  as  great  again,  if  not  twice  as  great,  as  that  of  the  dark 
space  between  the  inner  edge  of  the  bright  ring  and  the  plan- 
et. But  Iluyghens  describes  the  dark  space  as  about  equal 
to  the  breadth  of  the  ring,  or  a  little  greater.  Supposing  the 
ring  the  same  then  as  now,  could  this  error  have  arisen  from 
the  imperfection  of  his  telescope  ?  No  ;  because  the  effect  of 
the  imperfection  would  have  been  directly  the  opposite.  The 
old  telescopes  all  represented  planets  and  other  bright  objects 
too  large,  and  therefore  would  sliow  dark  spaces  too  small, 
owing  to  the  irradiation  produced  by  their  imperfect  glasses. 
A  strong  confirmation  of  Struve's  view  is  found  in  the  old 


CONSTITUTION  OF  THE  RING.  S49 

pictures  given  in  Fig.  89  by  those  observers  wlio  could  not 
clearly  make  out  the  ring.  In  nearly  all  cases  the  dark  spaces 
were  more  conspicuous  than  the  edges  of  the  ring.  But  if 
we  now  look  at  Saturn  tlirough  a  very  bad  atmosphere,  though 
the  elliptical  outline  of  the  ring  may  be  clearly  nuide  out, 
the  dark  space  will  be  almost  obliterated  by  the  encroachment 
of  the  light  of  the  planet  and  ring  upon  it.  The  question  is, 
therefore,  one  of  those  the  complete  solution  of  which  must 
be  left  to  future  observers. 

§  5.  Constitution  of  (lie  Ring. 

The  difficulties  which  investigators  have  met  with  in  ac- 
countino;  for  the  nngs  of  Saturn  are  of  the  same  nature  as 
those  we  have  described  as  arising  from  spectroscopic  discov- 
eries respecting  the  envelopes  of  the  sun.  They  illustrate  the 
philosophic  maxim  that  surprise — in  which  term  we  may  in- 
clude all  difiiculty  and  perplexity  which  men  meet  with  in 
seeking  to  account  for  the  phenomena  of  nature — is  a  result 
of  partial  knowledge,  and  cannot  exist  either  with  entire  ig- 
norance or  complete  knowledge.  Those  who  are  perfectly 
ignorant  are  surprised  at  nothing,  because  they  expect  noth- 
ing, while  perfect  knowledge  of  what  is  to  happen  also  pre- 
cludes the  same  feeling.  The  astronomers  of  two  centuries 
ago  saw  nothing  surprising  in  the  fact  of  a  pair  of  rings  sur- 
rounding a  planet,  and  accompanying  it  in  its  orbit,  because 
they  were  not  acquainted  with  the  effects  of  gravitation  on 
such  bodies  as  the  rings  seemed  to  be.  But  when  Laplace  in- 
vestigated the  subject,  he  found  that  a  homogeneous  and 
uniform  ring  surrounding  a  planet  could  not  be  in  a  state 
of  stable  equilibrium.  Let  it  be  balanced  ever  so  nicely,  the 
slightest  external  force,  the  attraction  of  a  satellite  or  of  a 
distant  planet,  would  destroy  the  equilibrium,  and  the  ring 
would  soon  be  precipitated  upon  the  planet.  He  therefore 
remarked  tliat  the  rings  must  have  irregularities  in  their 
form,  such  as  Ilerschel  supposed  he  had  seen;  but  he  did 
not  investigate  the  question  whether  with  those  irregularities 
the  equilibrium  would  really  be  stable. 


350  THE  SOLAR  STSTE 

The  question  was  next  taken  np  in  this  country  by  Profess- 
oi*s  Poirce  and  Bond.  The  latter  started  from  the  supposed 
result  of  observations  —  that  new  divisions  show  themselves 
from  time  to  time  in  the  ring,  and  then  close  up  again.  He 
thence  inferred  that  the  rings  must  be  fluid,  and,  to  confirm 
this  view,  he  showed  the  impossibility  of  even  an  irregular 
solid  pair  of  rings  fulfilling  all  the  necessary  conditions  of 
stability  and  freedom  of  motion.  Professor  Peirce,  taking  up 
the  same  subject  from  a  mathematical  point  of  view,  found 
that  no  conceivable  form  of  iri'egular  solid  ring  would  be  in  a 
state  of  stable  ecjuilibrium;  he  therefore  adopted  Bond's  view 
that  the  rings  were  fiuid.  Following  up  the  investigation, 
he  found  that  even  a  fiuid  ring  would  not  bo  entirely  stable 
without  some  external  support,  and  he  attributed  that  support 
to  the  attractions  of  the  satellites.  But  as  Laplace  did  not 
demonstrate  that  irregularities  would  make  the  ring  stable,  so 
Peirce  merely  fell  back  upon  the  attraction  of  the  satellites  as 
a  sort  of  forlorn  hope,  but  did  not  demonstrate  that  the  fiuid 
ring  would  really  be  stable  under  the  infiuence  of  their  attrac- 
tion. Indeed,  it  now  seems  very  doubtful  whether  this  at- 
tractioti  would  have  the  effect  supposed  by  Peirce. 

Tlie  next,  and,  we  may  say,  the  last,  important  step  was 
taken  by  Professor  J.  Clerk  Maxwell,  of  England,  in  the 
Adams  prize  essay  for  1856.  He  brought  forward  objections 
which  seem  unanswerable  against  botli  the  solid  and  the  fiuid 
ring,  and  revived  a  theory  propounded  by  Cassini  about  the 
beginning  of  the  last  century.*  This  astronomer  considered 
the  ring  to  be  formed  by  a  cloud  of  satellites,  too  small  to 
be  separately  seen  in  the  telescope,  and  too  close  together  to 
admit  of  the  intervals  between  them  being  visihle.  This  is 
the  view  of  the  constitution  of  the  rimjs  of  Saturn  now  most 
generally  adopted.  The  reason  why  the  ring  looks  solid  and 
continuous  is  that  the  satellites  are  too  small  and  too  numerous 
to  be  seen  singly.     They  are  like  the  separate  little  drops  of 


*  See  Memoirs  of  the  French  Academy  of  Sciences  for  1715,  p.  47;  or  Cas- 
sini's  "Clemens  d'Astronouiie,"i).  338,  Paris,  1740. 


THE  SATELLITES  OF  SATURN.  351 

water  of  which  clouds  and  fon;  are  composed,  which,  to  our 
eyes,  seeiri  like  solid  masses.  In  the  dusky  ring  the  particles 
may  be  so  scattered  that  we  can  see  through  the  cloud,  the 
reason  that  it  looks  dusky  being  simply  the  comparatively 
small  number  of  the  particles,  so  that  to  the  distant  eye  they 
appear  like  the  faint  stij>pling  of  an  engraving. 

The  question  arises  whether  the  comparative  darkness  of 
some  portions  of  the  bright  ring  may  not  be  due  to  the  paucity 
of  the  particles,  which  allows  the  dark  background  of  the  sky 
to  be  seen  through.  This  question  cannot  be  positively  an- 
swered until  furtlier  observations  are  made;  but  the  prejion- 
derance  of  evidence  favors  the  view  that  the  entire  brio-ht 
ring  is  opaque,  and  that  the  dark  shading  is  due  entirely  to  a 
darker  color  of  that  part  of  the  ring.  Indeed,  for  anytliing 
we  certainly  know,  the  whole  ring  may  be  continuous  and 
opaque,  the  darker  shade  of  some  parts  arising  solely  from  the 
particles  being  there  black  in  color.  The  only  way  to  settle 
conclusively  the  questions  whether  these  i>arts  of  the  ring  look 
black,  owing  to  the  sky  beyoiid  showing  through  openings,  as 
it  were,  or  from  a  black  color  of  the  ring,  is  to  find  whether  a 
star  or  other  object  can  be  seen  through  the  dark  s]mces.  Jhit 
an  opportunity  for  seeing  a  bright  star  through  the  ring  has 
never  yet  presented  itself.  The  most  obvious  way  of  settling 
the  question  in  respect  to  the  dusky  ring  is  to  notice  whether 
the  planet  itself  can  bo  seen  through  it ;  but  this  is  much  more 
difficult  than  might  be  supposed,  owing  to  tlie  ill -defined  as- 
pect of  the  ring.  The  testimony  of  both  Lassell  and  Trouve- 
lot  is  in  favor  of  the  view  that  this  ring  is  partially  transpar- 
ent; but  their  observations  will  need  to  be  repeated  when  tlic 
ring  is  opened  out  to  our  sight  after  1882. 

§  6.  The  Satellites  of  Saturn. 

AVhen  Iluyghens  commenced  his  observations  of  Saturn  in 
1G55,  he  saw  a  star  near  the  planet  which  a  few  days'  observa- 
tion enabled  him  to  recognize  as  a  satellite  revolving  round  it 
in  about  fifteen  days.  In  his  "  S>/stema  Saturnium,'^  he  vent- 
ured to  express  the  opinion  that  this  discovery  completed  the 


352 


THE  SGLAR  SYSTEM. 


solar  system,  whicli  now  cojnprised  six  planets  (Saturn  being 
then  the  outermost  known  planet)  and  six  satellites  (one  of 
the  earth,  four  of  Jupiter,  and  this  one  of  Saturn),  making 
the  perfect  number  of  twelve.  He  was,  therefore,  confident 
that  no  mo''e  satellites  were  left  to  discover,  and  through  fail- 
ing to  search  for  odiers,  he  probably  lost  the  honor  of  addi- 
tional discoveries. 

Twelve  years  after  this  prediction,  Cassini  discovered  a  sec- 
ond satellite  outside  that  found  by  Huyghens,  and  within  a 
few  years  more  he  found  three  others  inside  of  it.  The  dis- 
covery of  fonr  satellites  by  one  astronomer  was  so  brilliant  a 
result  of  French  science  that  the  Government  of  France 
struck  a  medal  in  commemoration  of  it,  bearing  the  inscrip- 
tion Saiurni  Satellites  primum  cogniti.  These  five  satellites 
completed  the  number  known  for  more  than  a  century.  In 
1789  Ilerschel  discovered  two  new  ones  still  nearer  the  rma 
than  those  found  by  Cassini.  The  space  between  the  ring  and 
the  inner  one  is  so  small  that  the  satellite  is  generally  invisible, 
even  in  the  most  powerful  telescopes.  Finally,  in  Sei)tember, 
1848,  the  Messrs.  Bond,  at  the  Observatory  of  Harvard  Col- 
lege, found  an  eighth  satellite,  whila  examining  the  ring  of 
Saturn.  By  a  singular  coincidence,  this  satellite  was  found  by 
Mr.  Lassell,  of  England,  only  a  couple  of  nights  after  it  M'as 
detected  by  the  Bonds.  The  names  which  have  been  given  to 
these  bodies  are  shown  in  the  following  list,  in  which  the  sat- 
ellites are  arranged  in  the  order  of  their  distance  from  the 
planet.  The  distances  are  given  in  semidiameters  of  Saturn. 
More  exact  elements  will  be  found  in  the  Appendix  to  this 
volume. 


Distance  froin 

No. 
1 

Name. 

Plimet, 

Discoverer. 

Date. 

Mimns 

3.3 

Heiscliel. . 

1  78!»,  September  1  7tli. 

2 

Kiicehuliis. 

4.3 

Herscliel. . 

178!),  Angust  28tli. 

3 

Tetlivs 

r..3 

(.'assini .... 

1(!84,  Mnrcii. 

4 

Dioiie 

().8 

Cassini .... 

ni84,  March. 

i> 

Hlien 

n.r. 

C'as'<ini .... 

1(;72,  December  23(1. 

G 

Titan 

20.7 

Hnvglieii.T. 

Id')."),  March  2r)th. 

7 

Hvperion  . 

20.8 

Bond 

I8i8,  September  IGtli. 

8 

Jnpetus.,.. 

64.4 

Cassini .... 

1(171.  ()ctoi)er. 

VBANUS  AND   IW  SATELLITES.  353 

The  brightness,  or  rather,  the  visihiHty,  of  tliese  satellites 
follows  the  same  order  as  their  discovery.  The  smallest  tel- 
escope will  show  Titan,  and  one  of  very  moderate  size  will 
show  Japetus  in  the  western  part  of  its  orbit.  Fonr  or  live 
inches  apertnre  will  show  Khea,  and  pei'haps  Tethys  and  Di- 
one,  while  seven  or  eight  inches  are  re({uired  for  Enceladus, 
and  even  with  that  aperture  it  will  probably  be  seen  only  near 
its  greatest  elongation  from  the  planet.  Mimas  can  be  seen 
only  near  the  same  position,  unless  the  ring  is  seen  edgewise, 
and  will  then  re(piire  a  large  telescope,  probably  twelve  inches 
or  upwards.  Finall}',  Hyperion  can  be  recognized  only  with 
the  most  powerful  telescopes,  not  only  on  account  of  its  faint- 
ness,  but  of  the  difficulty  of  distinguishing  it  from  minute  stars. 

All  these  satellites,  except  Japetus.  revolve  very  nearly  in 
the  plane  of  the  ring.  Consequently,  when  the  edge  of  the 
rino;  is  turned  towards  the  earth,  the  satellites  seem  to  swing 
from  one  side  of  the  planet  to  the  other  in  a  straight  line,  run- 
ning along  the  thin  edge  of  the  ring,  like  beads  on  a  string. 
This  phase  affords  the  best  opportunity  of  seeing  the  inner 
satellites  Mimas  and  Enceladus,  because  they  are  no  longer 
obscured  by  the  brilliancy  of  the  ring. 

Japetus,  the  outer  satellite  of  all,  exhibits  this  remarkable 
peculiarity,  that  while  in  one  ])a.  o  of  its  orbit  it  is  the  bright- 
est of  the  satellites,  except  Titan,  in  the  opposite  part  it  is  al- 
most as  faint  as  Hyperion,  and  can  be  seen  oidy  in  large 
telescoj)es.  When  west  of  the  planet,  it  is  bright;  when  east 
of  it,  faint.  This  peculiarity  has  been  accounted  for  oidy  by 
supposing  that  the  satellite,  like  our  moon,  always  presents 
the  same  face  to  the  planet,  and  that  one  side  of  it  is  white 
and  the  other  intensely  black.  The  only  difficulty  in  the  way 
of  this  explanation  is  that  it  is  doubtful  whether  any  known 
substance  is  so  black  as  one  side  of  the  satellite  must  be  to 
account  for  such  great  changes  of  brilliancy. 

§  7.  ZIranus  and  its  Satellites. 

Uranus,  the  next  planet  beyond  Saturn,  is  at  a  mean  dis- 
tance from  the  sun  of  about  1770  millions  of  miles,  and  per- 

24 


354  THE  SOLAR  SYSTEM. 

forms  a  revolution  in  84  yeai's.  It  shines  as  a  star  of  the  sixth 
magnitude,  and  can  tlierefore  be  seen  with  the  naked  eye,  if 
one  knows  exactly  where  to  look  foi  it.  It  was  in  opposition 
February  lltli,  1877,  and  the  time  of  opposition  during  the 
remainder  of  the  present  century  may  be  found  by  adding  4^ 
days  for  every  year  subsequent  to  1877.  To  find  it  readily, 
either  with  a  telescope  or  the  naked  eye,  recourse  must  be  had 
to  the  JVautical  A  lma7uw,  where  the  ])Osition  (right  ascension 
and  declination)  is  given  for  each  day  in  the  year. 

Of  course  the  smallest  telescopes  will  show  this  planet  as  a 
star,  but  to  recognize  its  disk  a  magnifying  power  of  at  least 
100  should  be  used,  and  200  will  bo  necessary  to  any  one  who 
is  not  a  practised  observer.  As  seen  in  a  large  telescope,  the 
planet  has  a  decided  sea-green  color.  No  markings  have  ever 
been  certainly  seen  on  the  disk,  and  therefore  no  changes 
which  could  be  due  to  an  axial  rotation  have  ever  been  estab- 
lished ;  but  it  may  be  regarded  as  certain  that  it  does  rotate 
in  the  same  plane  in  which  the  satellites  revolve  around  it. 

Discovery  of  Uranus. —  This  planet  was  discovered  by  Sir 
William  Ilerschel,  in  March,  1781.  Perceiving  by  its  disk 
that  it  was  not  a  star,  and  by  its  motion  that  it  was  not  a  neb- 
ula, he  took  it  for  a  comet.  The  possibility  of  its  being  a  new 
planet  did  not  at  first  occur  to  him  ;  and  he  therefore  com- 
municated his  discovery  to  the  Royal  Society  as  being  one  of 
a  new  comet.  Various  computing  astronomers  thereupon  at- 
tempted to  find  the  orbit  of  the  supposed  comet,  from  the  ob- 
servations of  Ilerschel  and  others,  assuming  it  to  move  in  a 
parabola,  like  other  comets.  But  the  actual  motion  of  the 
body  constantly  deviated  from  the  orbits  thus  computed  to 
such  an  extent  that  new  calculations  had  to  be  repeatedly 
made.  After  a  few  weeks  it  was  found  that  if  it  moved  in  a 
pai'abola,  the  nearest  distan(;e  to  the  sun  must  be  at  least  four- 
teen times  that  of  the  earth  from  the  sun,  a  j>erihelion  distance 
many  times  greater  than  that  of  any  known  comet.  This  an- 
nouncement gave  the  hint  that  some  other  hypothesis  must  Ik; 
resorted  to,  and  it  was  then  found  that  ill  uie  observations 
could  be  well  re})resented  by  a  circular  orbit,  with  a  radius 


VBANVS  AND  ITS  SATELLITES.  355 

nineteen  times  tliat  of  the  earth's  orbit.    The  object  was,  there- 
fore, a  planet  moving  at  double  the  distance  of  Saturn. 

With  a  commendable  feeling  of  gratitude  towards  the  royal 
patron  who  had  afforded  him  the  means  of  making  his  dis- 
coveries, Ilerschel  pro}  osed  to  call  the  new  planet  Georgium 
Sidus  (the  Star  of  the  C  eorges).  This  name,  contracted  to  "  the 
Georgian,"  was  em[)loyed  in  England  until  1850,  but  never 
came  into  use  on  the  Continent.  1  alawde  thought  the  most 
appropriate  name  of  the  planet  was  that  of  its  discoverer,  and 
therefore  proposed  to  call  it  Ilersc'.el.  But  this  name  met 
with  no  more  favor  than  the  other.  Several  other  names  were 
proposed,  but  that  of  Uranus  at  length  met  with  universal 
adoption.  It  was  proposed  by  Bode  as  the  most  appropriate, 
on  the  ground  that  the  most  distant  body  of  our  system  might 
be  properly  named  after  the  oldest  of  the  gods. 

After  the  elliptic  orbit  of  the  planet  had  been  accurately 
computed,  and  its  path  mapped  out  in  the  heavens,  it  was 
found  that  it  had  been  seen  a  surprising  number  of  times  as  a 
star  without  the  observers  having  entertained  any  suspicion  of 
its  planetary  nature.  It  had  passed  through  the  field  of  their 
telescopes,  and  they  had  noted  the  time  of  its  transit,  or  its 
declination,  or  both,  but  had  entered  it  in  their  journals  simply 
as  an  unnamed  star  of  the  constellation  in  which  it  happened 
to  be  at  the  time.  It  had  been  thus  seen  five  times  by  Flam- 
steed,  the  first  observation  being  in  1090,  nearly  a  century  be- 
fore the  discovery  by  Ilerschel.  What  is  most  extraordina- 
ry, it  had  been  observed  eight  times  in  rapid  succession  by 
Le  MouTiier,  of  Paris,  in  December,  1768,  and  January,  1769. 
Had  that  astronomer  merely  taken  the  trouble  tr  reduce  and 
compare  his  observations,  he  would  have  anticipated  Ilerschel 
by  twei.'e  years.  Indeed,  considering  how  easily  the  planet 
can  be  seen  with  the  naked  eye,  it  is  illustrative  of  the  small 
amount  of  care  devoted  to  cataloG-uinfi:  tlic  stars  that  it  was 
not  discovered  without  a  telescope. 

SctlelUtes  of  Uranus.  —  In  January  and  February,  1787, 
Ilerschel  fc  nd  tliat  Uranus  was  accompanied  by  two  satel- 
lites, of  which  the  inner  performed  a  revolution  in  a  little  less 


35 G  THE  SOLAR  SYSTEM. 

thai)  nine  clays,  and  tlio  outer  in  thirteen  days  and  a  half. 
The  existence  of  these  two  satellites  was  well  authenticated 
by  his  observations,  and  they  have  been  frequently  observed 
in  recent  times.  They  can  be  seen  with  a  telescope  of  one- 
foot  aperture  or  upwards.  Afterwards  Ilerschel  made  a  very 
assiduous  search  for  other  satellites.  He  encountered  many 
difficulties,  not  only  from  the  extreme  faintness  of  the  objects, 
but  from  the  difficulty  of  deciding  whether  any  object  he 
might  see  was  a  satellite,  or  a  small  star  which  happened  to 
be  in  the  neighborhood.  He  at  length  announced  the  probable 
existence  of  four  additional  satellites,  the  orbit  of  one  being 
inside  of  those  of  the  two  certain  ones,  one  between  them,  and 
two  outside  them.  This  made  an  entire  number  of  six;  and 
though  the  evidence  adduced  bv  Ilerschel  in  favor  of  the  ex- 
istence  of  the  four  additional  ones  was  entirely  insufficient, 
and  their  existence  has  been  completely  disproved,  they  figure 
in  some  of  our  books  on  astronomy  to  this  day. 

For  half  a  century  no  telescope  more  powerful  than  that  of 
Ilerschel  was  turned  upon  Uranus,  and  no  additional  light  was 
thrown  upon  the  question  of  the  existence  or  non-existence  of 
the  questionable  objects.  At  length,  about  1846,  Mr.  William 
Lassell,  of  England,  constructed  a  reflector  of  two  feet  aper- 
ture, of  which  we  have  already  spoken,  and  of  very  excellent 
definition,  which  in  optical  power  exceeded  any  of  the  older 
instruments.  With  this  he  succeeded  in  discovering  two  new 
satellites  inside  the  orbits  of  the  two  brighter  ones,*  but  found 
no  trace  of  any  of  the  additional  satellites  of  Ilerscliel.  In  the 
climate  of  England,  he  could  make  only  very  imperfect  obser- 
vations of  these  bodies;  but  in  1852  he  moved  his  telescope 
temporarily  to  Malta,  to  take  advantage  of  the  purer  sky  of 
tliat  latitude,  and  there  lie  succeeded  in  determining  their  or- 
bits with  considerable  accuracy.  Their  times  of  revolution 
are  about  2^  and  4  days  resj)ectively.     They  may  fairly  be 

*  These  difficult  objects  were  also  sought  for  by  Otto  Stiuve  with  the  fifteen- 
inch  telescope  of  tiie  I'lilkowa  Observatory,  and  occasional  glimpses  of  them  were, 
he  believed,  attained  before  they  were  certaiidy  found  by  Mr.  Lassell,  but  he  was 
not  able  to  follow  them  so  continuously  as  to  fix  ujion  their  times  of  revolution. 


UJiANUS  AND  ITS  SATELLITES.  357 

regarded  as  the  most  difficult  known  objects  in  the  planetary 
system;  indeed,  it  is  only  with  a  few  of  the  most  powerful 
telescopes  in  existence  that  they  have  certainly  been  seen. 

The  non-existence  of  Ilerschel's  suspected  satellites  is  proved 
by  the  fact  that  they  have  been  sought  for  in  vain,  both  with 
Mr.  LasselTs  great  reflectors  and  with  the  Washington  twen- 
ty-six-inch refractor,  all  of  which  are  optically  more  powerful 
than  the  telescopes  of  llerschel.  There  may  be  additional 
satellites  which  have  not  yet  been  discovered  ;  but  if  so,  they 
must  be  too  faint  to  have  been  recognized  by  llerschel.  Pro- 
fessor Ilolden,  of  the  Naval  Observatory,  has  sought  o  show 
that  some  of  Ilerschel's  observations  of  his  supposed  inner  sat- 
ellites were  really  glimpses  of  the  objects  aftervirds  discov- 
ered by  Mr.  Lassell.  This  he  has  done  by  calculating  tiie  po- 
sitions of  these  inner  satellites  from  tables  for  the  date  of 
each  of  Ilerschel's  observations,  and  comparing  them  wiiii  the 
position  of  the  object  noted  by  llerschel.  In  four  cases,  the 
agreement  is  sufficiently  close  to  warrant  the  belief  that  ller- 
schel actually  saw  the  real  satellites;  but  Mr.  Lassell  attril)utes 
these  coincidences  to  chance,  and  contests  Professor  Ilolden's 
views. 

The  most  remarkable  peculiarity  of  the  satellites  of  Uranus 
is  the  great  inclination  of  their  orbits  to  the  ecliptic.  Instead 
of  beinir  inclined  to  it  at  small  auii-les,  like  the  orbits  of  all 
the  other  planets  and  satellites,  they  are  nearly  perpendicular 
to  it;  indeed,  in  a  geometrical  sense,  they  are  more  than  per- 
pendicular, because  the  direction  of  the  motion  of  the  satel- 
lites in  their  orbits  is  retrograde.  To  change  the  i)osition  of 
the  orbit  of  an  ordinary  satellite  into  that  of  the  orbits  of 
these  satellites,  it  would  have  to  be  tipped  over  100° ;  so  that, 
supposing  the  orbit  a  horizontal  plane,  the  point  correspond- 
ing to  the  zenith  would  be  10°  below  the  horizon,  and  the  up- 
per surface  would  be  inclined  beyond  the  perpendicular,  so  as 
to  be  the  lower  of  the  two  surfaces. 

Observations  of  the  satellites  afford  the  only  accurate  way 
of  determining  the  mass  of  Uranus;  because, of  the  adjoining 
planets,  Saturn  and  Neptune,  the  observations  of  the  first  are 


358  THE  SOLAR  SYSTEM. 

too  uncertain  and  those  of  the  last  too  recent  to  give  any  cer- 
tain result.  Measures  made  with  the  great  Washington  tele- 
scope show  this  mass  to  be  uiriT777 1  ^  result  which  is  probably 
correct  within  ^^■^y  part  of  its  whole  amount.''^ 

§  8.  Neptune  and  its  /Satellite. 

The  discovery  of  this  planet  is  due  to  one  of  the  boldest  and 
most  brilliant  conceptions  of  modern  astronomy.  The  planet 
was  felt,  as  it  were,  by  its  attraction  upon  Uranus;  and  its  di- 
rection was  thus  calculated  by  the  theory  of  gravitation  before 
it  had  been  recognized  by  the  telescope.  An  observer  was 
told  that  if  he  pointed  his  telescope  towards  a  certain  point  in 
the  heavens,  he  would  see  a  new  planet.  He  looked,  and  there 
was  the  planet,  within  a  degree  of  the  calculated  place.  It  is 
difficult  to  imagine  a  more  strikinoj  illusti'ation  of  the  certain- 
ty  of  that  bran(;h  of  astronomy  which  treats  of  the  motions  of 
the  heavenly  bodies  and  is  founded  on  the  theory  of  gravi- 
tation. 

To  describe  the  researches  which  led  to  this  result,  we  shall 
iiave  to  go  back  to  1820.  In  that  year,  Bouvard,  of  Paris, 
prepared  improved  tables  of  Jupiter,  Saturn,  and  Uranus, 
which,  although  now  very  imperfect,  have  formed  the  basis  of 
most  of  the  calculations  since  made  on  the  motions  of  those 
bodies.  lie  found  that  while  the  motions  of  Jupiter  and  Sat- 
urn were  fairly  in  accord  with  the  theory  of  gravitation,  it 
was  not  so  with  those  of  Uranus.  After  allowing  for  the  per- 
turbations produced  by  the  known  planets,  it  was  impossii)le 
to  find  any  orbit  which  would  satisfy  both  the  ancient  and  the 
recent  observations  of  Uranus.  By  the  ancient  observations 
we  mean  those  accidental  ones  made  by  Flamsteed,  Le  Mon- 
nier,  and  others,  before  the  planetary  character  of  the  object 
was  suspected ;  and  by  the  recent  ones,  those  made  after  the 
discovery  of  the  planet  by  Ilerschel,  in  1781.  Bouvard,  there- 
fore, rejected  the  older  observations,  founding  his  tables  on  the 
modern  ones  alone ;  and  leaving  to  future  investigators  the 


♦  Washington  Observations  for  1873  :  Appendix. 


NEPTUNE  AND   ITS  SATELLITE.  35'J 

question  wliether  the  difficulty  of  reconciling  tlie  two  systems 
arose  from  the  inaccuiacy  of  the  ancient  observations,  or  from 
the  action  of  some  extraneous  influence  upon  the  planet. 

Only  a  few  years  elapsed,  when  the  planet  began  to  deviate 
from  the  tables  of  Bouvard.  In  1830  the  error  amounted  to 
20'';  in  1840,  to  90";  in  1844,  to  2'.  From  a  non- astro- 
nomical point  of  view,  these  deviations  were  very  minute. 
Had  two  stars  moved  in  the  heavens,  the  one  in  the  place 
of  the  real  planet,  the  other  in  that  of  the  calculated  planet, 
it  would  have  been  an  eye  of  wonderful  keenness  which 
could  have  distinguished  the  two  from  a  single  star,  even  in 
1844.  But,  magnilied  by  the  telescope,  it  is  a  large  and 
easily  measurable  quantity,  not  for  a  moment  to  be  neglect- 
ed. The  probable  cause  of  the  deviation  was  sometimes  a 
subject  of  discussion  among  astronomers,  but  no  very  definite 
views  respecting  it  seem  to  have  been  entertained,  nor  did 
any  one  express  the  decided  opinion  that  it  was  to  be  attrib- 
uted to  a  trans-Uranian  planet,  natural  as  it  seems  to  us  such 
an  opinion  would  have  been. 

In  1845,  Arao-o  advised  his  then  vonng  and  unknown  friend 
Leverrier,  whom  he  knew  to  be  an  able  mathematician  and 
an  expert  computer,  to  investigate  the  subject  of  the  motions 
of  Uranus.  Leverrier  at  once  set  about  the  task  in  tho  most 
systematic  manner.  The  first  step  was  to  make  sure  that  the 
deviations  did  not  arise  from  errors  in  Bouvard's  theory  and 
tables ;  he  therefore  commenced  with  a  careful  recomputation 
of  the  perturbations  of  Uranus  produced  by  Jupiter  and  Sat- 
urn, and  a  critical  examination  of  the  tables.  The  result  was 
the  discovery  of  many  small  errors  in  the  tables,  wiiicii,  liow- 
ever,  were  not  of  a  character  to  give  rise  to  the  observed  de- 
viations. 

The  next  question  was  whether  any  orbit  could  be  •"ssigned 
which,  after  making  allowance  for  tne  action  of  Jupiter  and 
Saturn,  would  represent  the  modern  observations.  The  an- 
swer was  in  the  negative,  the  best  orbit  deviating,  tirst  on  one 
side  and  then  on  the  other,  by  amounts  too  great  to  be  attrib- 
uted to  errors  of  observation.     Supposing  the  deviations  to  be 


3G0  THE  SOLAR  SYSTEM. 

due  to  the  attraction  of  some  unknown  planet,  Levern'er  next 
inquired  where  this  planet  nnist  be  situated.  Its  orbit  cuiild 
not  lie  between  those  of  Saturn  and  IJraiuis,  because  then  it 
would  disturb  the  tnotions  of  Saturn  as  well  as  those  of  Uranus. 
Outside  of  Uranus,  therefore,  the  j)lanet  must  be  looked  for, 
and  probably  at  not  far  from  double  the  distance  of  that 
body;  this  being  the  distance  indicated  by  the  law  of  Titius. 
Complete  elements  of  the  orbit  of  the  unseen  planet  were 
linally  deduced,  making  its  longitude  325°  as  seen  from  the 
earth  at  the  beginning  of  1847.  This  conclusion  was  reached 
in  the  suuimer  of  184G. 

Leverrier  was  not  alone  in  reachino;  this  result.  In  1843, 
Mr.  John  C.  Adams,  then  a  student  at  Cambridge  University, 
England,  having  learned  of  the  discordances  in  the  theory  of 
Uranus  from  a  report  of  Professor  Airy,  attacked  the  same 
problem  which  Leverrier  took  hold  of  two  years  later.  In 
October,  1845,  he  communicated  to  Professor  Airy  elements 
<^f  the  planet  so  near  the  truth  that,  if  a  search  had  been  made 
with  a  large  telescope  in  the  direction  indicated,  the  ])lanet 
could  hardly  have  failed  to  be  found.  Tlie  Astronomer  Iloyal 
was,  however,  somewhat  incredulous,  and  deferred  his  search 
for  further  ex])lanations  from  Mr.  Adatns,  which,  from  some 
unexplained  (lause,  he  did  not  receive.  Meanwhile  the  planet, 
which  had  been  in  opposition  about  the  middle  of  August, 
was  lost  in  the  rays  of  the  sun,  and  could  not  be  seen  before 
the  following  summer.  A  most  extraordiiuiry  circumstance 
was  that  nothing  was  immediately  published  on  the  subject  of 
Mr.  Adams's  labors,  and  no  effort  made  to  secure  his  right  to 
priority,  although  in  reality  his  researches  preceded  those  of 
Leverrier  by  nearly  a  year. 

In  the  sunnner  of  1846,  M.  Leverrier's  elements  appeared, 
and  the  coincidence  of  his  results  with  those  of  Mr.  Adams 
was  so  striking,  that  Professor  Challis,  of  the  Cambridge  Ob- 
servatory, commenced  a  vigorous  search  for  the  planet.  Un- 
fortunately, he  adopted  a  mode  of  search  which,  although  it 
nuide  the  discovery  of  the  planet  certain,  was  extremely  la- 
borious.    Instead  of  endeavoring  to  recognize  it  by  its  disk, 


NEPTUNE  AND    ITS  SATELLITE.  301 

lie  sought  to  detect  it  l)y  its  motion  among  the  stars  —  a 
coui'do  wliieh  required  all  the  stars  in  the  neighhorhood  to 
have  their  ])ositioiis  rei)oate(lly  determined,  so  as  to  find 
wliieh  of  them  had  changed  its  positit)n.  Observations  of 
the  ])lanet  as  a  star  were  actually  made  on  August  4th,  1S40, 
and  again  on  August  12th ;  but  these  observations,  owing  to 
Mr.  (Miallis's  other  engagements,  were  not  reduced,  and  so  the 
fact  that  the  planet  was  obseived  did  not  api)ear.  His  mode 
of  proceeding  was  much  like  that  of  a  man  who,  knowing  that 
a  diamond  had  dropp(!d  near  a  certain  spot  on  the  sea-beach, 
should  remove  all  the  sand  in  the  neighborhood  to  a  conven- 
ient place  for  the  purpose  of  sifting  it  at  his  leisure,  and 
should  thus  have  the  diamond  actually  in  Ins  possession  with- 
out beiiiij  able  to  recoi!:nize  it. 

Early  in  September,  1840,  while  Professor  Challis  was  still 
working  away  at  his  observations,  entirely  unconscious  that 
rhe  great  object  of  search  was  securely  imprisoned  in  the  pen- 
cilled figures  of  his  note-book,  Leverrier  wrote  to  Dr.  Galle,  at 
Berlin,  suggesting  that  he  should  try  to  find  the  planet.  It 
happened  that  a  map  of  the  stars  in  the  region  occu[)ied  by 
the  planet  w'as  just  completed,  and  on  pointing  the  telescope 
of  the  Berlin  Observatory,  Galle  soon  found  an  object  which 
had  a  planetary  disk,  and  was  not  on  the  star  map.  Its  })osi- 
tion  was  carefully  determined,  and  on  the  night  following  il 
was  re-examined,  and  found  to  have  changed  its  place  among 
the  stars.  No  further  doubt  could  exist  that  the  long-souijht- 
for  planet  was  found.  The  date  of  the  optical  discovery  was 
September  23d,  1846.  The  news  reached  Professor  Challis 
October  1st,  and,  looking  into  his  note-book,  he  found  his  own 
observations  of  the  planet,  made  nearly  two  months  before. 

As  between  Leverrier  and  Adams,  the  technical  right  of 
priority  in  this  wonderful  investigation  lay  with  Leverrier,  al- 
though Adams  had  preceded  him  by  nearly  a  year,  for  the 
double  reason  that  the  latter  did  not  publish  his  results  before 
the  discovery  of  the  planet,  and  that  it  was  by  the  directions 
of  Leverrier  to  Dr.  Galle  that  the  actual  discovery  was  made. 
But  this  does  not  diminish  the  credit  due  to  Mr.  Adams  for 


o02  TUE  SOLAR  SYSTEM. 

his  boldness  in  attackini;,  and  his  skill  in  successfully  solviiif)^, 
so  noble  a  problem.  The  spirit  of  true  science  is  advancing 
to  a  stage  in  which  contests  about  priority  are  looked  upon  as 
below  its  dignity.  Discoveries  are  made  for  the  benefit  of 
mankind  ;  and  if  made  independently  by  sevei-al  persons,  it  is 
fitting  that  each  should  receive  all  the  credit  due  to  success  in 
iruiking  it.  We  should  consider  Mr.  Adams  as  entitled  to  the 
same  unqualified  admiration  which  is  due  to  a  sole  discoverer; 
and  whatever  claims  to  priority  he  may  have  lost  by  the  more 
fortunate  Leverrier  will  be  compensated  by  the  sym})athy 
which  must  ever  be  felt  towards  the  talented  vounff  student 
in  his  failure  to  secure  for  bis  work  that  immediate  publicity 
which  was  due  to  its  interest  and  importance. 

The  discovery  of  Neptune  gave  rise  to  a  series  of  research- 
es, in  which  American  astronomers  took  a  distinguished  part. 
One  of  the  first  questions  to  be  considered  was  wbether  the 
planet  had, like  Uranus,  been  observed  as  a  star  by  some  i)re- 
vious  astronomer.  *  This  question  was  taken  up  by  Mr.  Sears  C. 
Walker,  of  the  Naval  Observatory.  A  few  months'  observa- 
tion sufficed  to  show  that  the  distance  of  the  planet  from  the 
sun  was  not  far  from  30  (the  distance  of  the  earth  being,  as 
usual,  unity),  and,  assuming  a  circular  orbit,  he  computed  the 
approximate  place  of  the  planet  in  past  years.  lie  traced  its 
course  back  from  year  to  year  in  order  to  find  whether  at  any 
time  it  passed  througli  a  region  which  was  at  the  same  time 
being  swept  by  the  telescopes  of  observers  engaged  in  prepar- 
in<i:  cataloojues  of  stars.  lie  was  not  successful  till  he  reached 
the  year  1795.  On  the  8th  and  10th  of  May  of  that  yeai-, 
Lalande,  of  Paris,  had  swept  over  the  place  of  the  i)lanet.  It 
must  now  be  decided  whether  any  of  the  stars  observed  on 
those  nights  could  have  been  Neptune.  Although  the  exact 
place  of  the  planet  could  not  yet  be  fixed  for  an  epo(;h  so 
remote,  it  was  easy  to  mark  out  the  apparent  position  of  its 
orbit  as  a  line  among  the  stars,  and  it  must  then  have  been 
somewhere  on  that  line.  After  takino;  out  the  stars  which 
were  too  far  from  the  line,  and  those  which  had  been  seen  bv 
subsequent  observers,  there  renu\ii:ed  one,  observed  on   May 


NEPTUNE  AND  ITS  SATELLITE.  3{Jo 

10th,  which  was  very  near  the  computed  orbit.  Walker  at 
once  ventured  on  the  bold  {)rediction  that  if  this  re<^ion  of 
the  heavens  were  examined  with  a  telescope,  (/i<il  slur  would 
be  found  missing,  lie  connuunicated  this  opinion  ofHcially 
to  Lieutenant  Maury  and  other  scientitic  men  in  AV^ashin<rton, 
ij.nd  asked  that  the  search  might  be  made.  On  the  fir.  ^lear 
evening  the  examination  was  made  by  Professor  Hubbard, 
and,  surely  enough,  the  star  was  not  there. 

There  was,  however,  one  weak  point  in  the  conclusion  that 
this  was  really  the  planet  Neptime.  Lalande  had  marked  his 
observation  of  the  missing  star  with  a  colon,  to  indicate  that 
there  was  a  doubt  of  its  accuracy :  therefore  it  was  possible 
that  the  record  of  the  su})posed  star  might  have  been  the  sim- 
ple result  of  some  error  of  observation.  Happily,  the  original 
manuscripts  of  Lahinde  were  carefully  preserved  at  the  Paris 
Observatory ;  and  as  soon  as  the  news  of  Walker's  researches 
reached  that  city  an  examination  of  the  observations  of  May 
8th  and  10th,  1795,  was  entered  upon.  The  extraordinary  dis- 
covery was  made  that  there  was  no  mark  of  uncertainty  in  the 
original  record,  but  that  Lalande  had  observed  the  planet  both 
on  the  8th  and  10th  of  May.  The  object  having  moved  sliglit- 
ly  during  the  two  days'  interval,  the  observations  did  not 
agree  ;  and  Lalande  supposed  that  one  of  them  must  be  wrong, 
entirely  unconscious  that  in  that  little  discrepancy  lay  a  dis- 
covery which  would  have  made  his  name  immortal.  Without 
further  examination,  he  had  rejected  the  first  observation,  and 
copied  the  second  as  doubtful  on  account  of  the  discrepancy, 
and  thus  the  pearl  of  great  price  was  dropped,  not  to  be 
found  again  till  a  half-century  had  elapsed. 

For  several  years  the  investigation  of  the  motion  of  tlie  new 
planet  was  left  in  the  hands  of  Mr.  Walker  and  Professor 
Peirce.  The  latter  was  the  first  one  to  compute  the  perturba- 
tions of  Neptune  pi-oduced  by  the  action  of  the  other  planets. 
The  results  of  these  computations,  together  with  Mr.  Walk- 
er's elements,  are  given  in  the  Proceedings  of  the  American 
Academy  of  Arts  and  Sciences. 

Plnjsical  Aspect  of  Xej^tune. — On  the  physical  appearance  of 


364:  THE  SOLAR  SYSTEM. 

this  planet  very  little  can  be  said.  In  the  largest  telescopes 
and  throngh  the  finest  atmosphere,  it  presents  the  appearance 
of  a  perfectly  roni'd  disk  abont  3"  in  diameter,  of  a  pale-bliie 
color.  No  markings  have  been  seen  npon  it.  When  first 
seen  by  Mr,  Lassell,  he  snspected  a  ring,  or  some  such  append- 
age ;  but  fnture  observations  under  more  favorable  circum- 
stances showed  this  suspicion  to  be  without  foundation.  To 
recognize  the  disk  of  Neptune  with  ease,  a  magnifying  power 
of  300  or  upwards  must  be  employed. 

Satellite  of  Neptune. — Soon  after  the  discovery  of  Neptune, 
Mr.  Lassell,  scrutinizing  it  with  his  two-foot  reflector,  saw  on 
various  occasions  a  point  of  light  in  the  neighl)orhood.  Dur- 
ing the  following  year  it  proved  to  be  a  satellite,  having  a  pe- 
riod of  revolution  of  about  5  days  21  hours.  During  1847 
and  181-8  the  satellite  was  observed,  both  at  Cambridge  by  the 
Messrs.  Bond,  and  at  Pulkowa  by  Struve.  These  observations 
showed  that  its  orbit  was  inclined  about  30°  to  the  ecliptic, 
but  it  was  impossible  to  decide  in  which  direction  it  was  mov- 
ing, since  there  were  two  positions  of  the  orbit,  and  two  di- 
rections of  motion,  in  which  the  apparent  motion,  as  seen  from 
the  earth,  would  be  the  same.  After  a  few  years  the  change 
in  the  direction  of  the  planet  enabled  this  question  to  be  de- 
cided, and  showed  that  the  motion  was  retrograde.  The  case 
was  more  extraordinary  than  that  of  the  satellites  of  Uranus, 
since,  to  represent  both  the  position  of  the  orbit  and  the  di- 
rection of  motion  in  the  usual  way,  the  orbit  would  have  to  be 
tipped  over  150° ;  it  is,  in  fact,  nearly  upside  down.  The  de- 
terminations of  the  elements  of  the  satellite  have  been  ex- 
tremely discordant,  a  circumstance  w^hich  we  must  attribute 
to  its  extreme  faintness.  It  is  a  minute  object,  even  in  the 
most  powerful  telescopes. 

Measures  of  the  distance  of  the  satellite  from  the  planet, 
made  with  the  great  Washington  telescope,  show  the  mass  of 
Neptune  to  be  yv^inr-  "^l^e  mass  deduced  from  the  perturba- 
tions of  Uranus  is  TirrTro"?  '^"  agreement  as  good  as  could  be 
expected  in  a  quantity  so  dithcnlt  to  determine. 


ASPECTS  AND  FORMS  OF  COMETS.  365 


CHAPTER  Y. 

COMETS    AND    METEORS. 

§  1.  As2')ects  and  Forms  of  Comets. 

The  celestial  motions  which  we  have  hitherto  described 
take  place  with  a  majestic  uniformity  which  lias  always  im- 
pressed the  mil.  ^s  of  men  with  a  sense  of  the  imcliangeable- 
ness  of  tl.e  heavens.  But  this  nniformity  is  on  some  occasions 
broken  by  the  apparition  of  objects  of  an  extraordinary  as- 
pect, which  hover  in  the  heavens  for  a  few  days  or  weeks,  like 
some  supernatural  visitor,  and  then  disappear.  We  refer  to 
comets,  bodies  which  have  been  known  from  the  earliest  times, 
but  of  which  the  nature  is  not  yet  deprived  of  mystery. 

Comets  briglit  enough  to  be  noticed  with  the  naked  eye 
consist  of  three  parts,  which,  however,  are  not  completely  dis- 
tinct, but  run  into  each  other  by  insensible  degrees.  These 
are  the  nucleus,  the  coma,  and  the  tail. 

The  nucleus  is  the  bright  centre  whicih  to  the  eyy  presents 
the  appearance  of  an  ordinary  star  or  planet.  It  would  liaid- 
ly  excite  remark  but  for  the  coma  and  tail  by  which  it  is  ac- 
companied. 

The  coma  (which  is  Latin  for  hair)  is  a  mass  of  cloudy  or 
vaporous  appearance,  wliich  surrounds  the  nucleus  on  all  sides. 
Next  to  the  nucleus,  it  is  so  bright  as  to  l;e  hardly  distinguish- 
able from  it,  but  it  gradually  shades  off  in  every  direction. 
Nucleus  and  coma  combined  present  the  appcaranc^e  of  a  star, 
more  or  less  briglit,  shining  through  a  small  patcli  of  fog,  and 
are  together  called  the  head  of  the  comet. 

The  tail  is  a  continuation  of  the  coma,  and  consists  of  a 
stream  of  milky  light,  growing  wider  and  fainter  as  it  recedes 
from  the  comet,  until  the  eye  can  no  longer  trace  it.     A  curi- 


366  THE  SOLAR  SYSTEM. 

ous  feature,  noticed  from  the  earliest  times,  is  tliat  the  tail  is 
always  turned  from  the  sun.  The  extent  of  the  tail  is  very 
different  in  diffei-ent  comets,  that  appendage  being  brighter 
and  lonu'er  the  more  brilliant  the  comet.  Sometimes  it  miijht 
almost  escape  notice,  \vhilo  in  nuiny  great  comets  recorded  in 
history  it  has  extended  half-way  across  the  heavens.  The 
actual  length,  when  one  is  seen  at  all,  is  nearly  always  many 
millions  of  miles.  Sometimes,  though  rarely,  the  tail  of  tlie 
comet  is  split  up  into  several  branches,  extending  out  in 
slightly  different  directions. 

Such  is  the  general  appearance  of  a  comet  visible  to  the 
naked  eye.  When  the  heavens  were  carefully  swept  with  tel- 
escopes, it  was  found  that  comets  thus  visible  formed  but  a 
small  fraction  of  the  whole  number.  If  a  diligent  search  is 
kept  np,  as  many  coniets  are  sometimes  found  with  the  tele- 
scope in  a  si?igle  year  as  would  be  seen  in  a  lifetime  with  the 
unaided  eye.  These  "telescopic  comets"  do  not  always  pre- 
sent the  same  aspect  as  those  seen  witli  the  naked  eye.  The 
coma,  or  foggy  light,  generally  seems  to  be  developed  at  the 
expense  of  the  nucleus  and  the  tail.  Sometimes  either  no 
nucleus  a,t  all  can  be  seen  with  the  telescope,  or  it  is  so  faint 
and  ill-defined  as  to  be  hardly  distinguishable.  In  the  cases 
of  such  comets,  it  is  generally  impossible  to  distinguish  the 
coma  from  the  tail,  the  latter  being  either  entirely  invisible, 
or  only  an  elongation  of  the  coma.  Many  well-known  comets 
consist  of  hardly  anything  but  a  patch  of  foggy  light  of  more 
or  less  irregular  form. 

Notwithstanding  these  great  apparent  differences  betweeji 
the  large  comets  and  the  telescopic  ones,  yet,  when  we  close- 
ly watch  their  respective  modes  of  development,  we  find  them 
all  to  belong  to  one  class.  The  differences  are  like  those  be- 
tween some  animals,  which,  to  the  ordinary  looker-on,  have 
nothiuij  in  connnon,  but  in  which  the  zooloii-ist  sees  that  every 
part  of  the  one  has  its  counterpart  in  the  other — indeed,  tJie 
analogy  between  what  the  astronomer  sees  in  the  growth  of 
comets  and  the  zoologist  in  tlie  growth  of  animals  is  quite 
worthy  of  remark.     As  a  general  rule,  all  comets  look  nearly 


ASPECTS  AM)    FOIiMS   OF  COMETS. 


367 


Flo.  ao. — Views  of  Kiicke't)  comet  iu  ISTl,  by  Dr.  Vofjel, 

jilike  when  tliey  first  come  witliin  reach  of  tlie  telescope,  tlie 
suljseqneiit  diversities  arising  from  the  different  devolopments 
of  corresponding  parts.  The  first  appearance  is  that  of  a  lit- 
tle foggy  patch  withont  any  tail,  and  very  often  without  any 
visible  nucleus.  Thus,  in  the  case  of  Donati's  comet  of  1858, 
one  of  the  most  splendid  on  recoi-d,  it  was  more  than  tw{» 
months  after  the  first  discovery  before  there  was  any  a})pcp.i- 


368  THE  SOLAR  SYSTEM. 

aucc  of  a  tail.  To  enable  the  reader  to  see  the  relation  of 
this  to  a  very  diffused  teleseopic  comet,  we  present  a  teleseopic 
view  of  the  head  of  this  great  comet  when  near  its  brightest, 
and  three  drawings  of  Encke's  comet,  made  by  Dr.  Vogel,  in 
November  and  December,  1871. 

When  the  nucleus  of  a  telescopic  comet  begins  to  show  it- 
self, it  is  commonly  on  the  side  farthest  from  the  sun.  Sev- 
eral little  branches  will  then  be  seen  stretched  out  in  the  di- 
rection of  the  sun,  so  that  it  will  ai)pear  as  if  the  comet  had 
a  small  fan-shaped  tail  directed  towards  the  sun,  instead  of 
from  it,  as  is  usual.  Thus,  in  the  pictures  of  Encke's  comet 
in  Figs.  1  and  2,  the  sun  is  towards  the  left,  and  we  see  what 


Fiu.  91.— Head  of  Uouiiti's  great  comet  of  la'iS,  after  Bond. 

looks  like  three  little  tails,  the  middle  one  pointed  towards  the 
sun.  But  if  we  look  at  the  view  of  Donati's  comet,  Fig.  1)1, 
we  see  several  little  lines  branching  npwards  from  the  centre 
of  the  head,  and  it  is  to  these,  and  not  to  the  tail,  that  the  lit- 
tle tails  in  the  figures  of  Encke's  comet  correspond.  In  fact, 
the  general  rule  is  that  the  heads  of  comets  have  a  fan-shaped 
structure,  the  handle  of  the  fan  being  in  the  nucleus,  and  the 
middle  arm  pointing  towards  the  sun  ;  and  it  is  this  append- 
age which  hrst  shows  itself. 

In  the  larger  comets,  this  fan  is  surrounded  by  one  or  more 


MOTIONS,  ORIGIN,  AND  NUMBER   OF  COMETS.  369 

semicircular  arclics,  or  envelopes,  the  inner  one  forming  its 
curved  border;  but  tins  arch  does  not  show  itself  in  very  faint 
comets.  The  true  tail  of  the  comet,  when  it  appears,  is  always 
directed  from  the  sun,  and  therefore  away  from  the  fan.  In 
Fig.  90,  No.  3,  a  very  faint  true  tail  will  be  seen  extending 
out  towards  the  lower  right-liand  corner  of  the  picture,  which 
was  opposite  to  tlie  direction  of  the  sun.  On  the  other  hand, 
though  the  branchcb  turned  towards  the  sun  have  disa))peared, 
the  fan-like  form  can  st'U  be  traced  in  the  head.  In  Fig.  91, 
the  true  tail  is  turned  downwards :  owing  to  the  large  scale  of 
the  })icture,  only  the  commencement  of  it  can  be  seen.  The 
central  line  of  the  tail,  it  will  be  remarked,  is  comparatively 
dark.     This  is  very  generally  the  case  with  bright  comets. 

§  2.  Motions.,  Origin^  and  Numhe.r  of  Comets. 

AVhen  it  was  found  by  Kei)ler  that  all  the  planets  moved 
around  the  sun  in  conic  sections,  and  when  Newton  sh(>wed 
that  this  motion  was  the  necessary  result  of  the  gravitation  of 
the  platiets  towards  the  sun,  the  question  naturally  arose  wheth- 
er comets  moved  according  to  the  same  law.  It  was  found  by 
Newton  that  the  comet  of  1080  actually  did  move  in  such  an 
orbit,  but  instead  of  being,  like  the  planetary  orbits,  nearly 
circular,  it  was  very  eccentric,  being  to  all  appearance  a  pa- 
rabola. 

A  parabola  being  one  of  the  orbits  which  gravitation  would 
cause  to  be  described,  it  was  thus  made  certain  that  comets 
gravitated  towards  the  sun,  like  planets.  It  was,  however,  im- 
possible to  say  whether  the  orbit  was  really  a  parabola  or  a 
very  elongated  ellipse.  The  reason  of  this  difficulty  is  that 
comets  are  visil)le  in  only  a  very  small  portion  of  their  orbits, 
quite  close  to  the  sun,  and  in  this  portion  the  forms  of  a  })a- 
rabola  and  of  a  very  eccentric  ellipse  are  so  nearly  the  same, 
that  they  cannot  always  be  distinguished. 

Tliere  is  this  very  important  difference  between  an  elliptical 
and  a  parabolic  orbit  —  that  the  former  is  closed  up,  and  a 
comet  moving  in  it  tnust  come  back  some  time,  whereas  the 
two  branches  of  the  latter  extend  out  into  infinite  space  with- 

25 


370 


THE  SOLAR  SYSTEM. 


out  ever  meeting, 


A  comet  moving  in  a  parabolic  orbit  will, 
therefore,  never  return,  but,  after  once  sweeping  past  the  sun, 
will  continue  to  recede  into  infinite  space  forever.  The  same 
thing  will  happen  if  the  comet  moves  in  an  liyperbola,  which  is 


I 

\ 


Parabolic  nrbit.  Eccentric  elllpsd. 

Fici.  92.— Parabolic  ami  elliptic  orbit  of  a  comet.  The  comet  is  invisible  in  the  dotted  part 
of  the  orbits,  and  the  forms  of  the  viHible  parts',  a,  h,  cannot  be  disstinjjiiislied  in  the 
two  orbitf.  But  the  ellipse  forms  a  closed  curve,  while  the  two  branches  of  the  pa- 
rabola continue  forever  without  meeting. 

the  third  class  of  orbit  that  may  be  described  under  the  influ- 
ence of  gravitation.  In  a  parabola,  the  slightest  retardation 
of  a  comet  would  change  the  orbit  into  an  ellipse,  the  velocit}' 
being  barely  sufficient  to  carry  the  comet  off  forever,  whereas 
in  an  hyperbola  there  is  more  or  less  velocity  to  spare.  Thus 
the  parabola  is  a  sort  of  dividing  curve  between  the  hyperbola 
and  the  ellipse. 

The  astronomer,  knowing  the  position  of  an  orbit,  can  tell 
exactly  what  velocity  is  necessary  at  any  point  of  it  in  order 
that  a  body  moving  in  it  may  go  off,  never  to  return.  A  body 
thrown  from  the  earth's  surface  with  a  velocity  of  seven  miles 


MOTIONS,  ORIGIN,  AND  NUMBER   OF  COMETS.  371 

a  second,  and  not  retarded  by  the  atmosphere,  would  never 
return  to  the  earth,  but  would  describe  some  sort  of  an  orbit 
round  the  bun.  It  would,  in  fact,  be  a  little  planet.  If  the 
earth  M-ere  out  of  the  way,  a  body  moving  past  the  earth's 
orbit  at  the  rate  of  twenty-six  miles  a  second  would  have  just 
the  velocity  necessary  to  describe  a  parabola.  If  the  velocity 
of  a  comet  exceeds  this  limit  at  that  point  of  its  orbit  which 
is  92|^  millions  of  miles  from  the  sun,  then  the  comet  must 
go  off  into  infinite  space,  never  to  return  to  our  system.  But 
with  a  less  velocity  the  comet  must  be  brought  back  by  the 
sun's  attraction  at  some  future  time,  the  time  being  longer  the 
more  nearly  the  velocity  reaches  twenty-six  miles  per  second. 
It  is  by  the  velocity  that  the  astronomer  must,  in  general,  de- 
termine the  form  of  the  orbit.  If  it  corresponds  exactly  to 
the  calculated  limit,  the  orbit  is  a  parabola;  if  it  exceeds  this 
limit,  it  is  an  hyperbola;  if  it  falls  short  of  it,  it  is  an  ellipse. 

Now,  in  the  large  majority  of  comets  the  velocity  is  so  near 
the  parabolic  limit  that  it  is  not  possible  to  decide,  from  ob- 
servations, whether  it  falls  short  of  it  or  exceeds  it.  In  the 
case  of  a  few  comets  the  observations  indicate  an  excess  of 
velocity,  but  an  excess  is  so  minute  that  its  reality  camiot  be 
confidently  asserted.  It  cannot,  therefore,  be  said  with  cer- 
tainty that  any  known  comet  revolves  in  a  hyperbolic  orbit, 
and  thus  it  is  possible  that  all  comets  belong  to  our  system, 
and  will  ultimately  return  to  it.  It  is,  however,  certain  that 
in  the  majority  of  cases  the  return  will  be  delayed  many  cen- 
turies, nay,  perhaps  many  thousand  years.  There  are  quite  a 
number  of  comets  which  are  known  to  be  periodic,  returning 
to  the  sun  at  regular  intervals  in  elliptic  orbits.  Some  o. 
these  have  been  observed  at  several  returns,  so  that  their  exact 
period  has  been  determined  with  great  certainty :  in  the  case 
of  others,  the  periodicity  has  been  inferred  only  from  the  fact 
that  the  velocity  fell  so  far  short  of  the  parabolic  limit  that 
there  could  be  no  doubt  of  the  fact  that  the  comet  moved  in 
an  ellipse. 

In  this  (piestion  of  cometary  orbits  is  involved  the  very  in- 
teresting one,  whether  comets  should  be  considered  as  belong- 


372  THE  SOLAR  SYSTEM. 

ing  to  our  system,  or  as  mere  visitors  from  the  stellar  rpaoes. 
We  may  conceive  of  them  as  stray  fragments  of  original  neb- 
ulous matter  scattered  through  the  great  wilderness  of  space 
around  us,  drawn  towards  our  sun  one  by  one  as  the  long  ages 
elapse.  If  no  planets  surrounded  the  sun,  or  if,  surrounding 
it,  they  were  immovable,  a  comet  thus  draM'n  in  M'otdd  whirl 
around  the  sun  in  a  i)arabolic  orbit,  and  leave  it  again,  not  to 
return  until  millions  of  years  had  elapsed,  because  the  veloci- 
ty it  would  acquire  by  falling  towards  the  sun  would  be  just 
sufficient  to  carry  it  back  into  the  infinite  void  fi-om  which  it 
came.  But  owing  to  the  motions  of  the  several  planets  in 
their  orl)its,  the  comet  would  have  its  velocity  changed  in 
passing  each  of  them,  the  change  being  an  acceleration  or  a 
retardation,  according  to  the  way  in  which  it  passed.  If  the 
total  accelerations  produced  by  all  the  planets  exceeded  the 
retai'dations,  the  comet  would  leave  onr  system  with  more 
than  the  parabolic  velocity,  and  would  certainly  never  return. 
If  the  retarding  forces  chanced  to  be  in  excess,  the  oi'bit 
would  be  changed  into  an  ellipse  more  or  less  elongated,  ac- 
cording to  the  amount  of  this  excess.  In  the  large  majority 
of  cases,  the  retardation  would  be  so  slight  that  the  most  del- 
icate observations  could  not  show  i'',  and  it  could  be  know^n 
only  by  calculation,  or  by  the  return  of  the  comet  after  tens 
or  hundreds  of  thousands  of  years.  But  should  the  comet 
chance  to  pass  very  near  a  planet,  especially  a  large  planet 
like  Jupiter,  the  retardation  might  be  so  great  as  to  make  the 
comet  revolve  in  an  orbit  of  quite  short  period,  and  thus  be- 
come a  seemingly  permanent  member  of  our  system.  So  near 
an  approach  of  a  comet  to  a  planet  would  not  be  likely  to  oc- 
cur more  than  once  in  a  number  of  centuries,  but  every  time 
it  did  occur  there  would  be  an  even  chance  for  an  additional 
comet  of  short  period,  the  orbit  of  which  would,  at  first,  al- 
most intersect  that  of  the  planet  which  had  deranged  it.  It 
might  not,  however,  be  a  known  comet,  because  the  orbit 
might  be  wholly  beyond  the  reach  of  our  vision. 

It  is  impossible,  in  the  present  state  of  science,  to  say  with 
certainty  whether  the  periodic  comets  were  thus  brought  into 


MOTIONS,  OliiaiN,  AND  NUMBER   OF  COMETS.  373 

our  system  ;  but  it  seems  probable  that  tliey  were,  from  the 
fact  that  many,  if  not  all,  of  the  orbits  of  these  comets  pass 
near  the  orbits  of  some  of  the  planets.  That  the  ])lanetary 
and  cometary  orbits  in  such  a  case  should  intersect  now  is  not 
to  be  expected,  because  both  would  change  by  the  secular 
variations  resulting  from  the  action  of  the  planets.  Future 
research  will  probably  throw  more  light  on  this  question. 

Namher  of  Comets. — It  was  the  opinion  of  Kepler  that  the 
celestial  spaces  were  as  full  of  comets  as  the  sea  of  fish,  only 
a  small  proportion  of  them  coming  within  the  range  of  our 
telescopes.  That  only  an  insigniUcant  fraction  of  all  existing 
comets  have  ever  been  observed,  we  may  regard  as  certain. 
lOwing  to  their  extremely  elongated  orbits,  they  can  be  seen 
only  when  near  their  perihelion,  and  as  it  is  probable  that  the 
period  of  revolution  of  the  large  majority  of  those  which  have 
been  observed  is  counted  by  thousands  of  yeai's — if,  indeed, 
they  ever  return  at  all — our  observations  must  be  continued 
for  many  thousand  years  before  we  have  seen  all  which  come 
within  range  of  our  telescopes.  It  is  also  probable  that  all 
which  can  ever  be  seen  will  be  but  a  small  fraction  of  the 
number  which  exist,  because  a  comet  can  seldom  be  seen  un- 
less its  perihelion  is  either  inside  the  orbit  of  the  earth,  or  but 
little  outside  of  it.  There  are  a  few  exceptions  to  the  rule 
that  only  such  comets  are  seen,  the  most  notable  one  being 
that  of  the  comet  of  1729,  which,  at  perihelion,  was  more  than 
four  times  the  earth's  distance  from  the  sun.  This  comet  nnist 
have  been  one  of  extraordinary  magnitude,  as  almost  every 
other  known  comet  would  have  disappeared  entirely  from  the 
most  powerful  telescopes  of  that  time,  if  placed  at  the  dis- 
tance at  which  it  was  observed. 

The  actual  number  of  comets  recorded  as  visible  to  the 
naked  eve  since  the  Christian  era  is  mven  in  the  table  on  the 
following  page.* 

*  Tliis  table  is  taken  at  second-hand,  principally  from  Arngo  ("Astronomic 
Populaire,"  livre  xvii.,  chap.  xv.).  Arago  mentions  but  eight  as  visible  dining 
the  eighteenth  century.  I  have  considered  the  number  thirty-six,  given  by  Klein, 
as  more  probable. 


374: 


THE  SOLAR  SYSTEM. 


Yean  of  our  Era. 

Niiiiitwr 
of  CollieU. 

Ycurii  of  our  Era. 

Niiiuber 
of  CoineU. 

From      0  to     100 

22 
23 
44 
27 
l(i 

2r> 

22 
U\ 
42 
2(5 

From 

1001  to  1 100 

3(; 

2() 

2<; 

2!» 
27 
31 
12 
3(5 
10 

"     101  "     200 

1101   ""   1200 

"    201  ♦'     3(K) 

1201    "   1300 

•'    301  "     400 

1301    "   1400 

1401    "  1")0() 

"    401  "    noo 

"  r.oi  "   (!oo 

laOl    "   KJOO 

"    GOl  "     700 

KiOl   "   1700 

"    701  "     800 

1701   "  1800 

"     801    "     1)00 

1801   "  1875 

"     !)()1  "    1000 

111  round  iiiiiiibers,  about  live  hundred  comets  visible  to  the 
naked  eye  have  been  recorded  since  our  era,  making  a  general 
average  of  one  every  four  years.  Besides  these,  nearly  two 
hundred  telescopic  comets  have  been  observed  since  the  in- 
vention of  the  telescope  ;  so  that  the  total  number  of  these 
bodies  observed  during  the  period  in  question  does  not  fall 
far  short  of  seven  hundred.  Several  new  telescopic  comets 
are  now  discovered  nearly  every  year,  the  number  sometimes 
ranging  up  to  six  or  eight.  It  is  probable  that  the  annual 
number  of  this  class  discovered  depends  very  largely  on  the 
skill,  assiduity,  and  good  -  fortune  of  the  astronomers  who 
chance  to  be  engaged  in  searching  for  them. 

§  3.  RemarJcahle  Comets. 

In  unenlightened  ages  comets  were  looked  on  with  terror, 
as  portending  pestilence,  war,  the  death  of  kings,  or  other 
calamitous  or  remarkable  events.  Hence  it  happens  that  in 
the  earlier  descriptions  of  these  bodies,  they  are  generally 
associated  with  some  contemporaneous  event.  The  descrip- 
tions of  the  comets  themselves  are,  however,  so  vague  and 
indefinite  as  to  be  entirely  devoid  of  either  instruction  or  in- 
terest, as  it  often  happens  that  not  even  their  course  in  the 
heavens  is  stated. 

The  great  comet  of  1680  is,  as  already  said,  remarkable  for 
being  not  only  a  brilliant  comet,  but  the  one  by  which  New- 
ton proved  that  comets  move  under  tlie  influence  of  the  gravi- 
tation of  the  sun.  It  first  appeared  in  the  autumn  of  IGSO, 
and  continued  visible  most  of  the  time  till  the  following  spring. 


REMAliKAIiLE  COMETS.  375 

It  fell  down  almost  in  a  direct  line  to  the  sun,  passing  nearer 
to  that  luminary  than  any  comet  before  known.  It  passed  its 
perihelion  on  December  18th,  and,  sweeping  round  a  largo 
arc,  went  back  in  a  direction  not  very  different  from  that  from 
which  it  came.  The  observations  have  been  calculated  and 
the  orbit  investigated  by  many  astronomers,  beginning  with 
Newton ;  but  the  results  show  no  certain  deviation  from  a 
parabolic  orbit.  Hence,  if  the  comet  ever  returns,  it  is  only 
at  very  long  intervals,  llalley,  however,  suspected,  with  some 
plausibility,  that  the  period  might  be  575  years,  from  the  fact 
that  great  comets  had  been  recorded  as  appearing  at  that  in- 
terval. Tiie  lirst  of  these  appearances  was  in  the  month  of 
September,  after  Julius  Caesar  was  killed;  the  second,  in  the 
year  531 ;  the  third,  in  February,  HOG ;  while  that  of  1(580 
made  the  fourth.  If,  as  seems  not  impossible,  these  were  four 
returns  of  one  and  the  same  comet,  a  fiftl  return  will  be  seen 
by  our  posterity  about  the  year  2255.  Until  that  time  the 
exact  period  must  remain  doubtful,  because  observations  made 
two  centuries  ago  do  not  possess  the  exactitude  which  will 
decide  so  delicate  a  point. 

Halleys  Comet. — Tavo  years  after  the  comet  last  duscribed, 
one  appeared  which  has  since  become  the  most  celebrated  of 
modern  times.  It  was  first  seen  on  August  19th,  1682,  and 
observed  about  a  month,  when  it  disappeared.  llalley  com- 
puted the  position  of  the  orbit,  and,  comparing  it  with  previ- 
ous orbits,  found  that  it  coincided  so  exactly  with  that  of  a 
comet  observed  by  Kepler  in  1607,  that  there  could  be  no 
doubt  of  the  identity  of  the  two  orbits.  So  close  were  they 
togetlier  that,  if  drawn  on  the  heavens,  the  naked  eye  would 
almost  see  them  joined  into  a  single  line.  The  chances  against 
two  separate  comets  moving  in  the  same  o>'bit  were  so  great 
that  llalley  could  not  doubt  that  the  comet  of  1682  was  the 
same  that  had  appeared  in  1607,  and  that  it  therefore  revolved 
in  a  very  elliptic  orbit,  returning  about  every  seventy-iive  years. 
His  conclusion  was  confirmed  by  the  fact  tliat  a  comet  was 
observed  in  1531,  which  moved  in  apparently  the  same  orbit. 
Again  subtracting  the  period  of  seventy-iive  years,  it  was 


370  THE  SOLAR  SYSTEM. 

found  that  the  comet  had  appeared  in  1450,  wlicn  it  spread 
such  terror  throughout  Cliristendoni  that  Pope  Calixtus  or- 
dered pi'ayers  to  be  offered  for  protection  against  the  Turks 
and  the  comet.  Tliis  is  supposed  to  be  the  circumstance  which 
gave  rise  to  tlie  po])uhir  myth  of  tlie  Pope's  Pull  against  the 
Comet. 

Tliis  is  tlie  earliest  occasion  on  which  observations  of  the 
course  of  the  comet  were  made  with  such  accuracy  that  its 
orbit  could  be  determined.  If  we  keep  subtracting  75^  years, 
we  shall  lind  that  we  sometimes  fall  on  dates  when  the  appa- 
rition of  a  comet  was  recoided ;  but  without  any  knowledge 
of  the  orbits  of  these  bodies,  it  cannot  be  said  with  certainty 
that  they  are  identical.  However,  in  the  returns  of  1450, 
1531,  1007,  and  1082,  at  nearly  equal  intervals,  llalley  had 
good  reason  for  predicting  that  the  comet  would  return  again 
about  1758.  This  gave  the  mathenuiticians  time  to  investi- 
gate its  motions ;  and  the  establishment,  in  the  mean  time,  of 
the  theory  of  gravitation  showed  them  how  to  set  about  the 
work.  It  was  necessary  to  calculate  the  effect  of  the  attrac- 
tion of  the  planets  on  the  motion  of  the  comet  during  the  en- 
tire seventy-six  years.  This  immense  labor  was  performed  by 
Clairaut,  who  found  that,  in  consecpience  of  the  attractions  of 
Jupiter  and  Saturn,  the  return  of  the  comet  would  be  delayed 
018  days,  so  that  it  would  not  reach  its  perihelion  until  the 
middle  of  April,  1759.  Not  having  time  to  finish  his  calcula- 
tions in  the  best  wav,  he  considered  that  this  result  was  uncer- 
tain  by  one  month.  The  comet  actually  did  pass  its  perihelion 
at  midnight  on  March  12th,  1759. 

Seventy-six  years  more  were  to  elapse,  and  the  comet  would 
again  appear  about  1835.  Meanwhile,  great  improvements 
were  made  in  the  methods  of  computing  the  effects  of  planet- 
ary attraction  on  the  motions  of  a  comet,  so  that  mathemati- 
cians, without  expending  more  labor  than  Clairaut  did,  were 
enabled  to  obtain  much  more  accurate  results.  The  Frencli 
were  still  the  leading  nation  of  the  world  in  this  sort  of  inves- 
tigation, and  the  computation  of  the  return  of  the  comet  was 
undertaken  independently  by  two  of  their  leading  astronomers, 


REMARKABLE  COMETS. 


377 


Do  Daiiioiseau  and  Do  Pontecoulant.  Of  these,  the  lirst  an- 
nounced that  it  wonkl  reach  its  perihelion  on  Noveniher  4th, 
1885;  while  De  Pontecoulant,  after  revising  his  coniputations 
with  more  exact  deterniinations  of  the  masses  of  the  planets, 
assigned  Novenibc"'  13th,  at  2  a.m.,  as  the  date.  The  expected 
comet  was,  of  course,  looked  for  with  the  greatest  assiduity, 
and  was  iirst  seen  on  August  5th.  Approacliing  the  sun,  it 
passed  its  perihelion  on  November  10th,  at  eleven  o'clock  in 
the  morning,  only  three  days  after  the  time  predicted  by  De 
Pontecoulant. 

This  was  the  last  return  of  the  celebrated  comet  of  Ilalley. 
It  was  followed  until  May  17th,  1836,  when  it  disappeared 
from  the  sight  of  the  most  powerful  telescopes  of 'the  time, 
and  has  not  been  seen  since.  But  the  astronomer  can  follow 
it  with  the  eye  of  science  with  almost  as  much  certainty  as  if 
he  had  it  in  tlie  field  of  view  of  his  telescope.  We  cannot  yet 
fix  the  time  of  its  return  with  certainty ;  but  we  know  that  it 
reached  the  farthest  limit 
of  its  course,  which  ex- 
tends some  distance  be- 
yond the  01  bit  of  Nep- 
tune, about  1873,  and 
that  it  is  now  on  its  re- 
turn journey.  We  pre- 
sent a  diagram  of  its  or- 
bit, showing  its  position 
in  1874.  Its  velocity 
will  constantly  increase 
from  year  to  year,  and 

we      may      expect     it      to  F.«.  03.-Orbit  of  Ilalleys  comet. 

reach  perihelion  about  the  year  1911.  The  exact  date  caimot 
be  fixed  until  the  effect  of  the  action  of  all  the  planets  is  com- 
puted, and  this  will  be  a  greater  labor  than  before,  not  oidy 
because  greater  accuracy  will  be  aimed  at,  but  because  the 
action  of  more  planets  must  be  taken  into  account.  When 
Clairaut  computed  the  return  of  1759,  Saturn  was  the  outer- 
most known  planet.    When  the  return  of  1835  was  computed. 


378  THE  SOLAR  SYSTEM. 

I'^raniis  liad  been  added  to  the  list,  and  its  action  had  to  be 
taken  into  account.  Since  that  time  Neptune  has  beei'  dis- 
covered ;  and  tlie  astronomer  who  computes  the  return  of  1911 
must  add  its  action  to  that  of  the  other  planets.  By  doing  cO, 
we  may  hope  that  the  time  of  reaching  perihelion  will  be  pre- 
dicted within  one  or  two  days. 

TIte  Lost  BiekCs  Comet. — Nothing  could  more  strikingly  il- 
lustrate the  differencje  between  comets  and  other  heavenly 
bodies  than  the  fact  of  the  total  dissolution  of  one  of  the  for- 
mer. In  1826,  a  comet  was  discovered  by  an  Austrian  named 
Biela,  which  was  found  to  be  periodic,  and  to  have  been  ob- 
served in  1772,  and  again  in  1805.  The  time  of  i-evolution 
was  found  to  be  six  years  and  eight  months.  In  the  next  two 
returns,  the  earth  was  not  in  the  right  part  of  its  orbit  to  ad- 
mit of  observing  the  comet ;  the  latter  was  therefore  not  seen 
again  till  1845.  In  November  and  December  of  that  year 
it  was  observed  as  usual,  without  anything  remarkable  being 
noticed.  But  in  January  following,  the  astronomers  of  the 
Naval  Observatory  found  it  to  have  suffered  an  accident  nev- 
er before  known  to  happen  to  a  heavenly  body,  and  of  which 
no  explanation  has  ever  been  given.  The  comet  had  sepa- 
rated into  two  distinct  parts,  of  quite  unequal  brightness,  so 
that  there  were  two  apparently  complete  comets,  instead  of 
one.  During  the  month  following,  the  lesser  of  the  two  con- 
tinually increased,  until  it  became  equal  to  its  companion. 
Then  it  grew  smaller,  and  in  March  vanished  entirely,  though 
its  companion  was  still  plainly  seen  for  a  month  longer.  The 
distance  apart  of  the  two  portions,  according  to  the  computa- 
tions of  Professor  Hubbard,  was  about  200,000  miles. 

The  next  return  of  the  comet  took  place  in  1852,  and  was, 
of  course,  looked  for  with  great  interest.  It  Avas  found  still 
divided,  and  the  two  parts  were  far  more  widely  separated 
than  in  1840,  their  distance  having  increased  to  about  a  mill- 
ion and  a  half  of  miles.  Sometimes  one  part  was  the  bright- 
er, and  sometimes  the  other,  so  that  it  was  impossible  to  de- 
cide which  ought  to  be  regarded  as  representing  the  principal 
comet.     The  pair  passed  out  of  view  about  the  end  of  Sep- 


REMARKABLE   COMETS.  379 

teniber,  1852,  and  have  not  been  seen  since.  They  would, 
since  tlien,  have  made  three  conii)lete  revohitions,  returning  in 
1859,  1865,  and  1872.  At  the  iirst  of  these  returns,  the  rela- 
tive positions  of  the  comet  and  the  earth  were  so  unfavorable 
that  there  was  no  hope  of  seeing  the  former.  In  1865,  it 
could  not  be  fc^nd ;  but  it  was  thought  tliat  this  might  be  due 
to  the  great  distance  of  the  comet  from  us.  In  1872,  the  rela- 
tive positions  were  extremely  favorable,  yet  not  a  trace  of  the 
object  could  be  seen.*  It  had  seemingly  vanished,  not  into 
thin  air,  but  into  something  of  a  tenuity  compared  with  which 
the  thinnest  air  was  as  a  solid  millstone.  Some  invisible  frag- 
ments were,  however,  passing  along  the  comet's  orbit,  and  pro- 
duced a  small  meteoric  shower,  as  will  be  explained  in  a  later 
section. 

IVie  Great  Comet  of  184:3.  ~  This  remarkable  comet  burst 
suddenly  into  view  in  the  neighborhood  of  the  sun  about  the 
end  of  February,  1843.  It  was  visible  in  full  daylight,  so  that 
some  observers  actually  measured  the  angular  distance  be- 
tween the  comet  and  the  sun.  It  was  followed  until  the  mid- 
dle of  April.  The  most  remarkable  feature  of  the  orbit  of 
this  comet  has  been  already  mentioned :  it  passed  nearer  the 
sun  tl"".n  any  other  known  body  —  so  near  it,  in  fact,  that, 
with  a  very  slight  cliange  in  the  direction  of  its  original  mo- 
tion, it  would  actually  have  struck  it.  Its  orbit  did  not  cer- 
tainly deviate  from  a  parabola.  The  most  careful  investigation 
of  it — that  of  Professor  Hubbard,  of  Washington — indicated 
a  period  of  530  years  ;  but  the  velocity  whicli  would  ])r()ducc 
this  period  is  so  near  the  parabolic  limit  that  the  difference 
does  not  exceed  the  uncertainty  of  the  observations. 

Doiiatis  Comet  f/ 1858. — This  great  comet,  one  of  the  most 
magniticent  of  modern  times,  which  hung  in  the  western  sky 
during  the  autumn  of  1858,  will  be  well  remembered  by  all 
who  were  then  old  enough  to  notice  it.     It  was  first  seen  at 


*  Just  after  the  meteoric  shower,  Mr.  Pogson,  of  Miulras,  ohtuined  observa- 
tions of  an  object  which,  it  was  snjtposed,  might  have  been  a  fragment  of  this 
comet.  But  the  object  was  some  two  montlis  beliind  tlie  computed  position  of 
iho  comot,  so  that  tlie  identity  of  the  two  lias  never  been  accepted  by  astronomers. 


380  THE  SOLAR  SYSTEM. 

Florence,  on  June  2d,  1858,  by  Donati,  who  described  it  as  a 
very  faint  nebulosity,  about  3'  in  diameter.  About  the  end 
of  the  month  it  was  discovered  independently  by  three  Amer- 
ican observers  :  II.  P.  Tuttle,  at  Cambridge  ;  II.  M.  Parkhurst, 
at  Perth  Aniboy,  New  Jersey  ;  and  Miss  Maria  Mitchel,  at 
Nantucket.  During  the  tirst  three  months  of  its  visibility  it 
gave  no  indications  of  its  future  grandeur.  No  tail  was  no- 
ticed until  the  middle  of  August,  and  at  the  end  of  that 
month  it  was  only  half  a  degree  in  length,  while  the  comet 
itself  was  barely  visible  to  the  naked  eye.  It  continued  to 
approach  the  sun  till  the  end  of  September,  and  during  this 


FiQ.  94.— Great  comet  of  1868. 

month  developed  with  great  rapidity,  attaining  its  greatest 
brilliancy  about  the  first  half  of  October.  Its  tail  was  then 
40°  in  length,  and  10°  in  breadth  at  its  outer  end,  and  of  a 
curious  feather-like  form.  About  October  20th  it  paspc^d  so 
far  south  as  to  be  no  longer  visible  in  northern  latitudes ;  but 
it  was  followed  in  the  southern  hemisphere  until  March  fol- 
lowing. 

Observations  of  the  position  of  this  comet  soon  showed  its 
orbit  to  be  decidedly  elliptic,  with  a  period  of  about  2000 
years  or  less.  A  careful  investigation  of  all  the  observations 
was  made  by  Mr.  G.  W.  Hill,  who  found  a  period  of  1950 


ENCKE'S   COMET,  AND   THE  liESISTIXG    MEDIUM.      381 

years.  If  tin's  period  is  correct,  the  comet  must  liave  ai)peared 
about  ninety-two  years  before  our  era,  and  must  ap])ear  again 
about  tlie  year  3S08 ;  but  tlie  uncertainty  arisin<>:  from  tlie  im- 
perfections of  the  observations  may  amount  to  fifty  years. 

§  4.  Encl-es  Comet,  and  the  Resisting  Medium. 

Tlie  comet  which  in  recent  times  lias  most  excited  the  atten- 
tion of  astronomers  is  that  known  as  Eiicke's,  from  the  astron- 
omer who  first  carefully  investii^ated  its  motion.  It  was  first 
seen  in  January,  178G,  but  the  observations  only  continued 
through  two  days,  and  were  insufficient  to  determine  the  orbit. 
In  1705,  a  comet  was  found  by  Miss  Caroline  Ilerschel,  on 
which  observations  were  continued  about  three  weeks;  but  no 
very  accurate  orbit  was  derived  from  these  observations.  In 
1805,  the  same  comet  returned  again  to  perihelion,  but  its  iden- 
tity again  failed  to  be  recognized.  As  in  the  previous  returns, 
the  observations  continued  through  less  than  a  month.  It  was 
found,  for  the  fourth  time,  by  Pons,  of  Marseilles,  in  1818. 
When  its  orbit  was  calculated,  it  was  seen  to  coincide  so 
closely  Avitli  that  of  the  comet  of  1805  as  to  leave  no  doubt 
that  the  two  were  really  the  same  body,  But  the  first  astron- 
omers who  noticed  this  were  unal)le  to  decide  'whether  this 
was  its  first  return  since  1805,  or  Avhether  it  had  in  the  mean 
time  made  several  revolutions. 

The  motions  of  the  comet  were  now  taken  up  by  Encke,  of 
Berlin,  and  investigated  with  a  thoroughness  before  nnknown. 
He  found  the'  period  to  be  about  1200  days,  four  (!omi)lete 
revolutions  having  been  made  between  1805  and  1818.  Know- 
ing  this,  there  was  no  longer  any  ditticulty  in  identifying  the 
comet  of  1795  as  also  being  the  same,  three  complete  revolu- 
tions having  been  made  lietween  that  date  and  1805,  In  the 
intermediate  returns  to  perihelion,  its  position  had  been  so 
unfavorable  that  it  had  not  been  observed  at  all.  This  result 
was  received  bv  astronomers  with  the  greatest  interest,  be(;ause 
it  was  the  first  known  case  of  a  coir  t  of  short  period.  Its  re- 
turn in  1822  was  duly  predicted,  but  it  was  found  that  when 
near  its  greatest   brilliancy  it  would  be  visible  only  in  the 


382  THE  SOLAR  SYSTEM. 

soutlieni  hcniisj^here.  Happily,  Sir  Thomas  Brisbane  had  an 
observatory  at  Paramatta,  New  South  Wales,  and  his  assistant, 
Runiker,  was  so  fortunate  as  to  find  the  comet.  It  was  so 
near  the  position  predicted  by  Encke  that,  by  constantly  point- 
ing the  telescope  in  the  direction  predicted  by  thr.*^  astronomer, 
the  comet  was  in  the  field  of  view  during  its  whole  course. 

Encke  continued  to  investigate  the  course  of  the  comet  dur- 
ing  each  revolution  up  to  the  time  of  his  deatii,  in  1805.  At 
some  returns  it  could  not  be  seen,  owing  to  its  distance  from 
the  earth,  or  the  otherwise  unfavorable  position  of  our  planet ; 
but  generally  very  accurate  observations  of  'N  course  were 
made.  By  a  comparison  of  its  motions  wiiii  those  which 
would  result  from  the  gi'avitation  of  the  sun  and  planets,  he 
found  that  the  periodic  time  was  constantly  diminishing,  and 
was  thus  led  to  adopt  the  famous  hypothesis  of  Gibers,  that 
the  comet  met  with  a  resisting  medium  in  space.  The  dimi- 
nution of  the  period  was  about  two  hours  and  a  half  in  each 
revolution.  Tlie  conclusion  of  Encke  and  Olbers  was  that  the 
planetary  Ri)aces  are  filled  with  a  very  rare  medium — so  rare 
that  it  does  not  produce  the  slightest  effect  on  the  motion  of 
such  massive  bodies  as  the  planets.  The  comet  being  a  body 
of  extreme  tenuity,  probably  far  lighter  than  air,  it  might  be 
affected  by  such  a  medium.  The  existence  of  this  medium 
cannot,  however,  be  considered  as  established  by  Encke's  re- 
searches. In  the  first  place,  if  we  grant  the  fact  that  tlie 
time  of  revolution  is  continually  diminishing,  as  maintained 
by  the  great  German  astronomer,  it  does  not  follow  that  a  re- 
sisting medium  is  the  only  cause  to  which  we  can  attribute  it. 
But  the  main  point  is,  that  the  computations  on  which  Encke 
founded  his  hypothesis  are  of  such  intricacy  as  to  be  always 
liable  to  small  errors,  and  their  results  cannot  be  received 
with  entire  confidence  until  some  one  else  has  examined  the 
subject  by  new  and  improved  methods. 

Such  an  examination  is  now  being  made  by  Dr.  Von  Asten, 
of  Pulkowa ;  and,  although  it  is  still  unfinished,  it  seems  like- 
ly, in  the  end,  to  confirm  Encke's  results,  at  least  in  part.  Dr. 
Von  Asten  commenced  by  calculating  the  motion  of  the  comet 


ENCKE'S  COMET,  AND   THE  EESISTIXG   MEDIUM.       383 

from  the  theory  of  gravitation  during  the  period  from  1865 
to  1871,  within  which  the  comet  made  two  entire  revolutions, 
and  was  surprised  to  find  that  during  this  time  there  was  no 
deviation  from  the  computed  positions  w^hich  could  be  attrib- 
uted to  tlie  action  of  a  resistinij  medium.  But  on  carrvin<r 
the  calcuhition  back  to  1861,  lie  found  that  between  that  epocli 
and  1865  there  must  have  been  a  retarding  action  like  tliat 
supposed  by  Encke.  Carrying  his  work  forward  to  1875,  he 
found  that  between  1871  and  1875  there  was  once  more  evi- 
dence of  a  retardation  about  two-thirds  as  great  as  that  found 
by  Encke.  Tlie  absence  of  such  an  action  between  1865  and 
1871,  therefore,  seems  quite  exceptional,  and  difficult  of  ex- 
planation. 

To  judge  whether  the  deviations  in  the  motion  of  Encke's 
comet  are  really  due  to  a  resisting  medium,  wo  should  know 
whether  the  motions  of  other  comets  exhibit  similar  anom- 
alies. So  far  as  is  yet  known,  no  other  one  does.  There  is 
at  least  one  which  has  returned  a  sufficient  number  of  times, 
aud  of  which  the  motions  have  been  computed  with  sufficient 
care,  to  lead  to  an  entirely  definite  conclusion  on  this  point, 
namely,  the  periodic  comet  of  Faye,  which  has  been  investi- 
gated by  JVLiller.*  This  comet  was  discovei-ed  in  1848  by  the 
astronoMier  whose  name  it  bears,  and  was  soon  found  to  move 
in  an  elliptic  orbit,  with  a  period  of  a  little  more  than  seven 
years.  As  it  has  been  observed  at  several  returns  since,  Mtiller 
investigated  its  motions  with  a  view  of  finding  wliether  its 
period  was  affected  by  any  resisting  medium.  At  first  he 
thouirht  there  was  such  an  effect,  his  general  i-esult  beiui;  of 
the  same  nature  with  that  reached  by  Encke.  Kut  on  repeat- 
ing his  calculations  with  the  improved  data  afforded  by  a  first 
calculation,  he  found  that  the  result  arose  from  the  imperfec- 
tion of  the  latter,  and  that  the  comet  reallv  showed  no  sign  of 
a  change  in  its  mean  motion.  It  therefore  seems  certain  that, 
if  there  is  a  resisting  medium,  it  does  not  extend  out  far 
enough  from  the  sun  to  meet  the  orbit  of  Faye's  comet.     But 

♦  Professor  Axel  Miillor,  director  of  the  observatory  at  Lund,  Sweden. 


384  THE  SOLAR  SYSTEM. 

this  orbit  lies  wholly  outside  the  orbit  of  Mars;  so  tliat  if  the 
sun  were  surrouncled  by  an  atinospliere  extending  out  to  Mars, 
and  no  farther,  the  coniet  would  never  enter  it.  On  the  other 
liand,  Encke's  comet,  when  in  perihelion,  is  nearer  tlie  sun 
than  Mercury  is,  and  might  there  meet  a  resisting  r.iedium 
winch  did  not  extend  so  far  out  as  the  orbit  of  Mars.  AV^e 
must  therefore  adopt  one  of  two  conclusions:  either  the  cause 
which  is  supposed  to  affect  the  motion  of  Encke's  comet  is 
not  a  resisting  medium,  or,  if  it  is  such,  it  is  confined  to  the 
neighborhood  of  the  sun.  Considering  the  improlmbilit}'  of 
the  snn  having  any  atmosphere  which  can  extend  to  such  a 
distance,  the  formei  should  be  deemed  the  more  probable 
alterna<-'''e.  We  can  accept  it  the  more  readily,  from  the 
fact  that  comets  in  ceneral  exhibit  deviations  from  their  cal- 
culated  orbits  many  times  lai'ger  than  those  of  the  planets,  so 
that  an  exact  agreement  between  theory  and  observations  can 
never  be  expected  in  the  case  of  those  bodies. 

The  next  subject  to  which  we  would  ask  the  attention  of 
the  reader  is  that  of  the  physical  constitution  of  comets.  13ut 
this  subject  can  be  discussed  only  in  connection  with  another, 
to  which,  at  first  sight,  it  seems  to  have  no  relation,  thougli 
so  curious  a  relation  has  really  been  discovered  as  gi-eatly  to 
modify  our  views  of  what  a  comet  probably  is.  We  refer  to 
the  phenomena  of  meteoi*s,  meteoric  showers,  and  shooting- 
stars,  which  next  claim  our  attention. 

§  5.  Meteors  and  Slioothig-stars. 

If  we  carefully  watch  the  heavens  on  a  cloudless  night,  we 
shall  frequently  see  an  appearance  as  of  a  star  rajjidly  shoot- 
ing through  a  short  space  in  the  sky,  and  then  suddenly  dis- 
appearing. Three  or  four  such  shooting-stars  may  generally 
be  seen  in  the  course  of  an  hour.  Generally  they  are  visible 
only  for  a  second  or  two,  but  sometimes  move  slowly,  and  are 
seen  much  longer.  Occasionally  they  are  so  brilliant  as  to 
illuminate  the  whole  heavens,  and  they  are  then  known  as 
meteors — a  term  which  is  equally  applicable  to  the  ordinary 
shooting-stars.     In  general,  they  are  seen  only  one  at  a  time, 


METKOnS  A XI)  SiroOTrXG-STAIiS.  385 

and  are  so  minute  as  liardlv  to  attract  attention.  T'ut  thev 
liave  on  some  occasions  shown  themselves  in  such  nuinhers  as 
to  nil  the  heholdcrs  with  terror,  lest  the  end  of  tlie  world  had 
come.  The  Chinese,  Arahian,  and  other  historians  have  hand- 
ed down  to  us  many  .accounts  of  such  showers  of  meteors, 
which  have  been  brought  to  light  by  the  researches  of  Ed- 
ward Biot,  Quetelet,  Professor  II.  A.  Newton,  and  others.  As 
an  example  of  these  accounts,  we  give  one  from  an  Arabian 
writer : 

"  In  the  year  599,  on  the  last  day  of  Moharrem,  stars  shot 
hither  and  thitlier,  and  flew  against  each  other  like  a  swarm 
of  Ixuists;  this  phenomenon  lasted  until  daybreak;  people 
were  thrown  into  consternation,  and  made  supplication  to  the 
Most  Iligli :  there  was  never  the  like  seen  except  on  the  com- 
ing of  the  messenger  of  God,  on  whom  be  benediction  and 
peace." 

In  1799,  on  the  night  of  November  12th,  a  remarkable 
shower  was  seen  by  Humboldt  and  Bonpland,  who  wei'e  then 
on  the  Andes.  Humboldt  described  the  shower  as  commen- 
cing a  little  before  two  o'clock,  and  the  meteors  as  rising  above 
the  horizon  between  east  and  north-east,  and  moving  over  tow- 
ards the  south.  From  not  continuing  his  observations  lone: 
enough,  or  from  some  other  cause,  he  failed  to  notice  tliat  the 
lines  in  which  the  meteors  moved  all  seemed  to  converge  tow- 
ards the  same  point  of  the  heavens,  and  thus  missed  the  dis- 
covery of  the  real  cause  of  the  phenomenon. 

The  next  great  shower  was  seen  in  this  country  in  1833. 
All  through  the  Southern  States,  the  negroes,  like  the  Arabs  of 
a  previous  century,  thought  the  end  of  the  world  had  come  at 
last.  Tiie  ])henomenon  was  observed  ver}-  carefully  at  New 
Haven  bv  Professor  Olmsted,  who  worked  out  a  theory  of  its 
cause.  Although  his  ideas  are  in  many  respects  erroneous, 
they  were  the  means  of  suijijestins:  tlie  true  theory  to  others. 
The  recurrence  of  the  shower  at  this  time  suggested  to  the 
astronomer  (Jlbers  the  idea  of  a  thirty-four-year  period,  and 
led  him  to  predict  a  return  of  the  shower  in  1807.  A  few 
years  before  the  expected  time,  the  subject  was  taken  up  by 

26 


386  THE  SOLAR  SYSTEM. 

Professor  Newton,  of  Yale  College,  to  whose  researches  our 
knowledge  of  the  true  cause  of  the  phenomenon  is  very  large- 
ly due. 

The  phenomena  of  shooting-stars  branch  out  in  yet  another 
direction.  As  we  have  described  them,  they  are  seen  only  in 
the  higher  and  rarer  regions  of  the  atmosphere,  far  above  the 
clouds :  no  sound  is  heard  from  them,  nor  does  anytliing  reach 
the  surface  of  the  earth  from  which  the  nature  of  the  ol)ject 
can  be  inferred.  But  on  rare  occasions  meteors  of  extreme 
brilliancy  are  followed  by  a  loud  sound,  like  the  discharge  of 
heavy  artillery  ;  while  on  yet  rarer  occasions  large  masses  of 
metallic  or  stony  substances  fall  to  the  earth.  These  aerolites 
were  the  puzzle  of  philosojihers.  Sometimes  there  was  much 
scepticism  as  to  the  reality  of  the  })henomenon  itself,  it  ap- 
pearing to  the  doubters  more  likely  that  those  who  described 
such  things  were  mistaken  than  that  heavy  metallic  masses 
shonld  fall  from  the  air.  When  their  reality  was  placed  be- 
yond doubt,  many  theories  were  propounded  to  account  for 
tliem,  the  most  noteworthy  of  which  was  that  they  were 
thrown  from  volcanoes  in  the  moon.  The  problem  of  the 
motion  of  a  body  projected  from  the  moon  was  investigated 
by  several  great  mathematicians,  the  result  being  tliat  such  a 
body  could  not  rea(;h  the  earth  unless  projected  with  a  veloci- 
ty far  exceeding  anything  seen  on  our  planet. 

When  aerolites  were  examined  by  chemists  and  mineralo- 
gists, it  was  found  that  although  they  contained  no  new  chem- 
ical elements,  yet  the  combinations  of  these  elements  were 
quite  unlike  any  found  on  the  earth,  so  that  they  must  have 
originated  outside  the  earth.  Moreover,  these  combinations 
exhibited  certain  characteristics  peculiar  to  aerolites,  so  that 
the  mineralogist,  from  a  simple  examination  and  analysis  of 
a  substance,  could  detect  it  as  part  of  such  a  body,  though 
it  had  not  been  seen  to  fall.  Great  masses  of  matter  thus 
known  to  be  of  meteoric  origin  have  been  found  in  various 
parts  of  the  earth,  especially  in  Xorthern  Mexico,  where,  at 
some  unknown  period,  an  immense  shower  of  these  bodies 
seems  to  have  fallen. 


METEORS  AXI)  SHOOTING-STARS.  3S7 

Cause  of  Shootutg-stars. — It  is  now  universally  conceded  that 
the  celestial  spaces  are  crowded  with  innnineral)le  minute 
bodies  moving  around  the  sun  in  every  possible  khid  of  orbit. 
When  we  say  crowded,  we  use  the  word  in  a  relative  sense ; 
they  may  not  average  more  than  one  in  a  million  of  cubic 
miles,  and  yet  their  total  number  exceeds  all  calculation.  Of 
the  nature  of  the  minuter  bodies  of  this  class  nothing  is  cer- 
tainly known.  But  whatever  they  may  be,  the  earth  is  con- 
stantly encountering  them  in  its  motion  around  the  sun.  They 
are  burned  by  piassing  through  the  upper  regions  of  our  at- 
mosphere, and  the  shooting  -  star  is  simply  the  light  of  that 
burning.  AVe  shall  follow  Professor  Newton  in  calling  these 
in\islble  bodies  meteorokis. 

The  question  which  may  be  asked  at  this  stage  is.  Why  are 
these  bodies  burned  'I  Especially,  how  can  they  burn  so  sud- 
denly, and  with  so  intense  a  light,  as  to  be  visible  hundreds 
of  miles  away  ?  These  questioiis  were  the  stumbling-block  of 
investigators  until  they  were  answered,  clearly  and  conclusive- 
ly, by  the  discovery  of  the  mechanical  theory  of  heat.  It  is 
now  established  that  heat  is  oidv  a  certain  form  of  motion  : 
that  hot  air  differs  from  cold  air  only  in  a  more  rapid  vibra- 
tion of  its  molecules,  and  that  it  comnumicates  its  heat  to 
other  bodies  simply  by  striking  them  with  its  molecides,  and 
thus  setting  their  molecules  in  vibration.  Consequently,  if  a 
body  moves  rapidly  through  the  air,  the  impact  of  the  air 
upon  it  ought  to  heat  it  just  as  warm  air  would,  even  though 
the  air  itself  were  cold.  This  result  of  theory  has  been  ex- 
perimentally proved  by  Sir  William  Thomson,  who  found  that 
a  thermometer  placed  in  front  of  a  rapidly  moving  body  rose 
one  degree  when  the  body  moved  through  the  air  at  the  rate 
of  125  feet  per  second.  With  higher  velocities,  the  increase 
of  temperature  was  proportional  to  the  square  of  the  velocity, 
being  4  degrees  with  a  velocity  of  250  feet,  16  degrees  with 
one  of  500  feet  per  second,  and  so  on.  This  result  is  in  exact 
accordance  with  the  mechanical  theory  of  heat.  To  find  the 
effective  temperature  to  which  a  meteoroid  is  exposed  in  mov- 
ing through  our  atmosphere,  we  divide  its  velocity  in  feet  per 


388  THE  SOLAR  SYSTEM. 

second  by  125  ;  the  square  of  tlie  quotient  will  give  tlio  tem- 
perature in  degrees. 

Let  us  apply  this  principle  to  the  case  of  the  nieteoroids. 
Tlie  earth  moves  in  its  orbit  iit  the  rate  of  98,000  feet  ])er 
second  ;  and  if  it  met  a  meteoroid  at  rest,  our  atmosphere 
would  strike  it  with  this  velocity.  By  the  rule  we  have  given 
for  the  rise  of  temperature  (1)8,000  V  125^  =  78i'=  600,000 
degrees,  nearly.  This  is  many  times  any  temperature  ever 
produced  by  artificial  means.  If,  as  will  commonly  be  the 
case,  the  meteoroid  is  moving  to  meet  the  earth,  the  velocity, 
and  therefore  the  potential  temperature,  will  be  higher.  We 
know  that  the  meteoroids  which  produce  the  November  show- 
ers already  described  move  in  a  direction  nearly  opposite  that 
of  the  earth  with  a  velocity  of  26  miles  per  second,  so  that  the 
relative  velocity  with  which  the  meteoroids  meet  our  atmos- 
phere is  44  miles  per  second.  By  the  rule  we  have  given, 
this  velocity  corresponds  to  a  temperature  of  between  three 
and  four  million  de2;rees.  "VVe  do  not  mean  that  the  meteor- 
oids  are  actually  heated  up  to  this  temperature,  but  that  the 
air  acts  upon  them  as  if  it  were  heated  up  to  the  point  men- 
tioned ;  that  is,  it  burns  or  volatilizes  them  in  less  than  a  sec- 
ond with  an  enormous  evolution  of  light  and  heat,  just  as  a 
furnace  wonld  if  heated  to  a  temperature  of  three  million  de- 
grees. It  is  not  at  all  necessary  that  the  body  shoidd  be  com- 
bustible ;  the  light  and  heat  of  ordinary  burning  are  nothing 
at  all  compared  with  the  deflagration  \vhich  such  a  tempera- 
ture would  cause  by  acting  on  the  hardest  known  body.  A 
few  grains  of  platinum  or  iron  striking  the  atmosphere  with 
the  velocity  of  the  celestial  motions  nn'ght  evolve  as  much  light 
and  heat  as  are  emitted  by  the  burning  of  a  pint  of  coal-oil  or 
several  pounds  of  gunpowder ;  and  as  tlie  whole  operation  is 
over  in  a  second,  we  may  imagine  how  intense  the  light  must  be. 

The  varied  phenomena  of  aerolites,  meteors,  shooting-stars, 
and  meteoric  showers  depend  solely  on  the  number  and  nat- 
ure of  the  meteoroids  which  give  rise  to  them.  If  one  of 
these  bodies  is  so  large  and  firm  as  to  pass  through  the  atmos- 
phere and  reach  the  earth  without  being  destroyed  by  the  po- 


METEORS  ASD  SlIOOriNd-STAIlS.  381) 

tential  lieat,  wc  liave  an  acTolite.  As  this  passaijjo  only  occu- 
|»ios  a  few  seconds,  the  lieat  has  not  time  to  penetrate  far  into 
the  interior  of  the  body,  but  expends  itself  in  nieUing  and  vol- 
atihzing  the  outer  poitions.  When  the  body  lirst  strikes  the 
denser  portion  of  the  atmosphere,  tlie  resistance  becomes  so 
enormous  tliat  the  aeroHte  is  frecpiently  broken  to  pieces  with 
such  violence  that  it  seems  to  explode.  Furtlier  color  is  given 
to  the  idea  of  an  expU)sion  by  the  loud  detonation  which  fol- 
lows, so  that  the  explosion  is  frecpiently  spoken  of  as  a  fact, 
and  as  the  cause  of  the  detonation.  Ileally,  there  is  good  rea- 
son to  believe  that  both  of  these  phenomena  are  due  to  the 
body  striking  the  air  with  a  velocity  of  ten,  twenty,  or  thirty 
miles  a  second. 

If,  on  the  other  hand,  the  meteoroid  is  so  small  or  so  fusible 
as  to  be  dissii)ated  in  the  upper  regions  of  the  atmosphere,  we 
have  a  common  shooting-star,  or  a  meteor  of  greater  or  less 
l)rilliancy.  Very  careful  observations  have  been  made  from 
time  to  time,  with  a  view  of  iinding  the  height  of  these  bodies 
above  the  earth  at  their  appearance  and  disappearance.  An 
attempt  of  this  kind  was  made  by  the  Naval  Observatory  on 
the  occasion  of  the  meteoric  shower  of  November  13th,  18G7, 
when  Professor  Ilarkness  was  sent  to  Richmond  to  map  the 
paths  of  the  brighter  meteors  as  seen  from  that  point.  By 
comparing  these  paths  with  those  majiped  at  Washington,  the 
parallaxes,  and  thence  the  altitudes,  of  these  bodies  were  de- 
termined. The  lightning-like  rapidity  with  which  the  mete- 
oi-s  darted  tlu'ougli  their  course  rendered  it  impossible  to  ob- 
serve them  with  astronomical  precision  ;  hut  the  general  re- 
sult was  that  they  were  first  seen  at  an  average  heigiit  of  75 
miles,  and  disai>peared  at  a  height  of  55  miles.  Tliei-e  was 
no  positive  evidence  that  any  meteor  connncnced  at  a  height 
much  greater  than  100  miles.  It  is  remarkable  that  this  cor- 
responds  very  nearly  to  the  greatest  height  at  which  the  most 
brilliant  meteors  are  ever  certainly  seen.  These  ])henomena 
seem  to  indicate  that  our  atmosphere,  instead  of  terminating 
at  a  height  of  45  miles,  as  was  forinei'ly  su])posed,  really  ex- 
tends to  a  height  of  between  100  and  110  miles. 


390 


THE  SOLAR  SYSTEM. 


Tlie  ordinary  meteors,  wliich  wo  may  see  on  every  clear 
evening,  move  in  every  direction,  thus  showing  that  their  or- 
bits lie  in  all  possible  positions,  and  are  seemingly  scattered 
entirely  at  random.  But  the  case  is  quite  different  with  those 
nieteoroids  which  give  rise  to  meteoric  showers.  Here  we 
have  a  swarm  of  these  bodies,  all  moving  in  the  same  direc- 
tion in  parallel  lines.     If  we  mark,  on  a  celestial  globe,  the 


Fia.  95.— Meteor  paths,  illustrating  the  nidiaut  point. 

apparent  paths  of  the  meteors  which  fall  during  a  shower,  or 
if  we  suppose  them  marked  on  the  celestial  sphere,  and  then 
continue  them  backwards,  we  shall  find  them  all  to  meet  in 
the  same  point  of  the  heavens.  This  is  called  the  radiant 
point.  It  always  appears  in  the  same  position,  wherever  the 
observer  is  situated,  and  does  not  partake  of  the  diurnal  mo- 


RELATIONS  OF  COMETS  AND  METEOltOlDS.  391 

tion  of  the  eartli ;  that  is,  us  tlie  stars  seem  to  move  towards 
the  west  in  their  diurnal  course,  tlie  radiant  point  moves  with 
tliem.  The  point  in  (question  is  purely  an  effect  of  perspec- 
tive, being  the  "vanishing  point"  of  the  parallel  lines  in 
'  .ich  the  meteors  really  move.  These  lines  do  not  appear 
in  their  real  direction  in  6i)ace,  but  are  seen  as  projected  on 
the  celestial  sphere.  A  good  visible  illustration  of  the  effect 
in  question  may  be  afforded  by  looking  upwards  and  watch- 
ing falling  snow  during  a  calm.  The  flakes  which  are  fall- 
ing directly  towards  the  observer  do  not  seem  to  move  at  all, 
while  the  surrounding  flakes  seem  to  separate  from  them  on 
all  sides.  So  with  the  meteoric  showers.  A  meteor  coniinj' 
directly  towards  the  observer  does  not  seem  to  move  j>  ill, 
and  marks  the  radiant  i)oint  from  which  all  the  others  seem 
to  diverge.  The  great  importance  of  the  determination  of 
the  radiant  point  arises  from  the  fact  that  it  marks  the  direc- 
tion in  which  the  meteors  are  moving  relatively  to  the  eartii, 
and  thus  affords  some  data  for  determining  their  orbits. 

§  6.  Relations  of  Comets  and  Meteoroids. 

We  have  now  to  uiention  a  series  of  investiirations  which 
led  to  the  discovery  of  a  curious  connection  between  meteor- 
oids  and  comets.  These  investigations  were  commenced  by 
Professor  Newton  on  the  Xovomber  meteoric  showers.  Tra- 
cing back  the  historical  accounts  of  these  showers  to  which 
we  have  already  alluded,  he  found  that  the  tliirty-three-year 
period,  which  had  been  suspected  by  Olbers,  was  conflrmed  by 
records  reaching  back  a  thousand  vears.  Moreover,  the  show- 
ers  in  question  occurred  only  at  a  certain  time  of  the  year:  in 
1790  and  1833,  it  was  on  November  12th  or  November  13th. 
In  other  words,  the  shower  occurred  only  as  the  earth  passed 
a  certain  point  of  its  orbit.  But  this  point  was  found  not  to 
be  always  the  same,  the  showers  being  found  to  occur  about 
a  couple  of  days  earlier  every  century  as  they  were  traced 
back.  The  principal  conclusions  to  which  these  facts  led 
were  as  follows : 

1.  That  the  swarm  of  meteoroids  which  cause  the  Novera- 


393  THE  SOLAR  SYSTEM. 

ber  showers  revolve  around  the  sun  in  a  definite  orbit,  wliich 
intersects  the  orbit  of  the  earth  at  the  point  which  the  latter 
now  passes  on  Xoveniber  13tli. 

2.  The  point  of  intersection  of  the  two  orbits  moves  for- 
wards about  52"  per  ainuiin,  or  nearly  a  degree  and  a  half  a 
century,  owing  to  a  change  in  the  position  of  the  meteoric 
orbit. 

3.  The  swarm  of  meteoroids  is  not  equally  scattered  all 
around  their  orbit,  but  the  thickest  portion  extends  along 
about  one-lifteenth  of  the  orbit. 

4.  The  earth  meets  this  swarm,  on  the  average,  once  in 
33.25  years.  At  other  times  the  swarm  has  not  arrived  at 
the  point  of  crossing,  or  has  already  passed  it,  and  a  meteori(; 
shower  cannot  occur  unless  the  earth  and  the  swarm  cross  at 
the  same  time. 

Professor  Newton  did  not  definitely  determine  the  time  of 
revolution  of  the  meteors  in  their  orbit,  but  showed  that  it 
must  have  one  of  live  values.  The  greatest  of  these  values, 
and  the  one  which  it  seems  most  natural  to  select,  is  that  of 
the  mean  interval  between  the  showers,  or  33^  years.  Adopt- 
ing this  period,  it  would  follow  that  between  1799,  when 
Humboldt  saw  the  meteoric  shower,  and  1833,  when  it  was 
seen  throughout  the  United  States,  the  swarm  of  meteoroids 
had  been  Hying  out  as  far  as  the  planet  Uramis  in  a  very  el- 
liptical orbit,  and  returning  again.  But  the  periodic  time 
might  also  be  one  year  p.nd  about  eleven  days.  Then  the 
group  which  Humboldt  saw  on  November  12th,  1799,  would 
not  reach  the  same  point  of  its  orbit  until  November  23d, 
1800,  when  the  earth  would  have  passed  by.  Passing  11  days 
later  every  year,  it  would  make  about  33  revolutions  in  34 
years,  and  thus  would  pass  about  the  middle  of  November 
once  more,  and  another  shower  would  occur.  In  a  word,  giv- 
ing exact  numbers,  we  might  suppose  that  in  the  period  of 
33^  years  the  meteoroids  made  one  revolution,  or  32^,  34J, 
65^,  or  07^  revolutions,  and  the  conditions  of  the  problem 
would  be  equally  satisfied. 

At  the  same  time,  Professor  Newton  gave  a  test  by  which 


RELATIONS   OF  COMETS  AND   METEOROIDS.  393 

the  true  time  could  be  determined.  As  we  have  said,  he 
showed  that  the  node  of  the  orbit  changed  its  position  52"  a 
century,  and  there  could  be  no  doubt  that  this  change  was 
due  to  the  attraction  of  the  planets.  If,  then,  the  effect  of 
this  attraction  was  calculated  for  each  of  the  five  orbits,  it 
would  be  seen  which  of  thein  would  give  the  required  change. 
This  was  done  by  Professor  x\danis,  of  England,  and  the  result 
was  that  the  thirty-three-year  period,  and  that  alone,  was  ad- 
missible. 

These  researches  of  Professor  Newton  were  jiublished  in 
1864,  and  ended  with  a  prediction  of  the  return  of  the  shower 
on  Xovember  13th  of  one  or  more  of  the  three  followinij: 
years — probably  1866.  This  prediction  Mas  verified  by  a  re- 
markable meteoric  shower  seen  in  Europe  on  that  very  day, 
which,  however,  was  nearly  over  before  it  could  become  visi- 
ble in  this  country.  On  the  same  date  of  the  year  following, 
a  shower  was  visible  in  this  country,  and  excited  great  public 
interest.  From  the  data  derived  from  the  first  of  these  show- 
ers, Schiaparelli,  an  Italian  astronomer,  was  led  to  the  discovery 
of  a  remarkable  relation  between  meteoric  and  cometary  orbits. 
Assuming  the  period  of  the  November  meteoroids  to  be  33^ 
years,  he  computed  the  elements  of  their  orbit  from  the  ob- 
served position  of  the  radiant  point.  A  similar  com})utati()n 
was  made  by  Leverrier,  and  the  results  were  presented  to  the 
French  Acadeniy  of  Sciences  on  January  21st,  1867. 

The  exact  orbit  which  these  bodies  followed  through  space, 
crossing  the  earth's  orbit  at  one  point,  and  extending  out 
beyond  the  planet  Uranus  at  another,  was  thus  ascertained. 
But,  as  these  bodies  were  absolutely  invisible,  no  great  inter- 
est seemed  to  attach  to  their  orbit  until  it  was  found  that  a 
comet  was  moving  in  that  very  orbit.  This  was  a  faint  tele- 
scopic comet  discovered  by  Tempel,  at  Marseilles,  in  Decem- 
ber, 1865.  It  was  afterwards  independently  discovered  by 
Mr.  II.  P.  Tuttle,  at  the  Naval  Observatory,  Washington.  It 
passed  its  perihelion  in  January,  and,  receding  from  the  sun, 
vanished  from  sight  in  March.  It  was  soon  found  to  move 
in  an  elliptic  orbit,  but,  owing  to  the  uncertainty  of  observa- 


394 


THE  SOLAR  SYSTEM. 


tions  on  sncli  a  body, 
there  was  at  lirst  some 
disagreement  as  to  the 
exact  periodic  time. 
The  snbject  was  taken 
up  by  Dr.  Oppolzer,  of 
Vienna,  wlio,  in  Janu- 
ary, 1867,  was  able  to 
present  a  definitive  or- 
bit of  the  comet,  which 
was  published  in  the  As- 
tronomische  Nachricliten 
on  the  28th  of  that 
^  month.  We  now  pre- 
sent the  orbit  of  the 
comet,  as  found  by  Op- 
polzer, and  that  of  the 
meteors,  as  found  by 
Leverrier,  premising 
that  these  orbits  were 
computed  and  publish- 
ed within  a  few  days 

Fio.  96.— Oibit  of  November  meteors  and  the  comet    ^^    eacll    Other,   witllOUt 

°^  ^^^^-  any  knowledge  on  the 

part  of  either  astronomer  of  the  results  obtained  by  the  other : 


The  Cuniet. 

Meteoroids. 

Period  of  revolution 

33.18  V1&. 

O.ltO.H 

o.i>7(;r) 

1(52°  42' 
51°  2(5' 
42°  24' 

33.25  VIS. 

0.!)()U 

O.itHiK) 

105"   ID' 

51°  UV 

Near  node. 

Kccen  ti'ici  t  v 

Perihelion  distiMice 

Iiicliiiiilioi)  of  oioit 

Longitude  of  f!ie  node 

Longitude  of  perihelion 

The  similarity  of  these  orbits  is  too  striking  to  be  the  result 
of  chance.  The  only  element  of  which  the  values  differ  ma- 
terially is  the  inclination,  and  this  difference  proceeds  from 
Leverrier  not  having  used  a  very  exact  position  of  the  radiant 
point  in  making  his  computations.  Professor  Adams  found 
by  a  similar  calculation  that  the  inclination  of  the  orbit  of  the 


RELATIONS  OF  COMETS  AND  METEOROIDS. 


395 


meteoroids  was  163°  14',  only  half  a  degree  different  from  that 
of  the  orbit  of  Tenipel's  coinet.  The  result  of  these  investi"-a- 
tions  was  as  follows  : 

'The  November  meteoric  showers  arise  from  the  earth  encounterimj 
a  swarm  of  particles  following 
TempeVs  comet  in  its  orbit. 

When   this  *fact   came  out, 
Schiaparelli  had  been  working 
on  the  same  subject,  and  had 
come   to  a  similar  conclusion 
with  regard  to  another  group 
of  meteors.     It  had  lono-  been 
known  that  about  August  9th 
of  Qvevy  year  an  unusual  num- 
ber of  meteors  shoot  forth  from 
the  constellation  Perseus.     At 
times  these  showers  have  been 
infei-ior  only  to  those  of  No- 
vember.    Thus,  on  August  9th, 
1798,  they  succeeded  each  oth- 
er 80  rapidly  as  to  keep  the 
eye  of  the  observer  almost  con- 
stantly   engaged,   and    several 
hundred  may  nearly  always  be 
counted  on  the  nights  of  the 
9th,  10th,  and    Uth.      These 
August  meteors  are  remarka- 
ble in  that  they  leave  trails  of 
luminous    vapor   which    often 
last  several  seconds.     Assum- 
ing the  orbit  of 'this  group  to 
be  a  parabola,  it  was  calculated 
by  Scliiaparelli,  and  is  substan- 
tially the  same  with  that  of  a 
comet  observed  in  1862.     The 
following  are  the  elements  of 
the  orbits  of  the  two  bodies :        no.  9T.-orbit  of  the  third  comet  of  1802. 


396 


THE  SOLAU  SYSTEM. 


Cnlllet  11., 

M  tit  enrol  da. 

Perilielion  distiince 

0.!)(!2(i 

ii;5"  ;{-)' 

l;57'  27' 

y-14  41' 

0.!»(>4:{ 

115°  r.7' 
i;^8°  iG' 
a4;}°  28' 

Iiicliiuitiou  of  orbit 

Lonyitutle  of  the  node 

Longitude  of  tiie  ))eiilielioii 

It  appears  that  the  August  meteors  are  eaiis?d  by  a  long 
stream  of  bodies  following  the  second  comet  of  1802  in  its 
orbit,  or,  rather,  moving  in  the  same  orbit  with  it.  The  orbit 
of  this  comet  is  decidedly  elli[)tic ;  the  difference  from  the 
parabola  is,  however,  too  small  to  be  determined  with  great 
precision.  According  to  Oppolzer,  the  period  derived  from 
the  observations  would  be  124  years,  which,  however,  may  be 
ten  years  or  more  in  error. 

A  third  striking  case  of  the  connection  between  comets  and 
meteors  whicli  we  are  showing  is  afforded  by  the  actual  pre- 
diction of  a  meteoric  shower  on  the  nii>;ht  of  November  27th, 
1872.  I  have  already  described  Biela's  comet  as  first  break- 
ing into  two  pieces  and  then  entirely  disappearing,  as  though 
its  parts  had  become  completely  scattered.  This  is  one  of 
the  few  comets  which  may  come  very  near  the  earth,  the  lat- 
ter passing  the  orbit  of  the  comet  on  November  27th  of  each 
year.  By  calculation,  the  comet  should  have  passed  the  point 
of  crossing  early  in  September,  1872,  while  the  earth  reached 
the  same  ])oint  between  two  and  three  months  later.  Judg- 
ing from  analogy,  there  was  e\ery  reason  to  believe  that  the 
earth  would  encounter  a  stream  of  meteoroids  consisting  of  the 
remains  of  the  lost  comet,  and  that  a  small  meteoric  shower 
would  be  the  result.  Moi'eover,  it  was  shown  that  the  mete- 
el's  would  all  diverge  from  a  certain  point  in  the  constellation 
Andromeda,  as  the  radiant  point,  because  that  would  be  the  di- 
rection from  which  a  body  moving  in  the  orb't  of  the  comet 
would  seem  to  come.  The  prediction  was  fully  verified  in 
every  respect.  The  meteors  did  not  compare,  either  in  num- 
bers or  brilliancy,  with  the  great  displays  of  November ;  but, 
though  faint,  they  succeeded  each  other  so  rapidly  that  the 
most  casual  observer  could  not  fail  to  notice  thern,  and  they 
all  moved  in  the  predicted  direction. 


.RELATIONS  OF  COMETS  AND  METEOROWS.  397 

That  the  meteoroids  in  these  cases  originally  belonged  to 
the  comet,  few  will  dispute.  Accepting  this,  the  phenomena  of 
the  November  showers  lead  to  the  conclusion  that  tlie  comet 
of  1SG6,  with  which  they  are  associated,  was  not  an  original 
member  of  our  system,  bnt  has  been  added  to  it  within  a 
time  which,  astronomically  speaking,  is  still  recent.  Tiie  sep- 
arate meteoroids  which  form  the  stream  will  necessarily  have 
slightly  different  periodic  times.  Such  being  the  case,  they 
will,  in  the  course  of  many  revolutions,  gradually  scatter  them- 
selves around  their  entire  orbit;  and  then  we  shall  have  an 
equal  meteoric  shower  on  every  13tli  of  November.  This 
complete  scattering  seems  to  have  actually  taken  ])lace  in  the 
case  of  the  August  meteoroids,  since  we  have  nearly  the  same 
sort  of  shower  on  every  Otli  or  10th  of  August.  But  in  the 
case  of  the  November  meteors,  the  stream  is  not  yet  scattered 
over  one-tenth  of  the  orbit.  If  we  suppose  that  the  motions 
of  the  slowest  and  the  swiftest  bodies  of  the  stream  only  dif- 
fer by  a  thousandth  part  of  their  whole  amount — wliicli  is  not 
an  unreasonable  supposition — it  would  follow  that  the  stream 
had  oidy  made  about  100  revolutions  around  the  sun,  and  had 
therefore  been  rcvolvinoj  only  about  3300  vears.  Thourrli  this 
number  is  purely  hypothetical,  we  ma}'  say  ■with  confidence 
that  the  stream  has  not  been  in  existence  many  thousand 
years. 

This  opini(m  is  strongly  supported  by  the  fact  that  the  orbit 
of  this  meteoric  cotnet  passes  very  near  that  of  Uranus  as  well 
as  that  of  the  earth,  so  that  there  is  reason  to  believe  that  it 
was  introduced  into  our  system  by  the  attraction  of  one  of 
these  planets,  probably  of  Uranus.  If  the  comet  is  seen  on  its 
next  return,  in  1809,  we  may  hope  that  its  periodic  time  will 
be  detei'mined  with  sufficient  accuracv  to  enable  us  to  fix  with 
some  probability  the  exact  date  at  which  Ui-anus  brought  it 
into  our  system.  Indeed,  Leverrier  has  attempted  to  do  this 
already,  having  fixed  upon  the  year  1;20  of  our  era  as  the 
probable  date  of  this  event ;  but,  unfortunately,  neither  the 
position  of  the  orbit  nor  the  time  of  rex  Dlution  is  yet  known 
with  such  accuracy  as  to  inspire  confidence  in  this  result. 


898  THE  SOLAR  SYSTEM. 

Tlie  idea  that  tin's  November  group  is  soinetliing  coinpara- 
tivcly  new  is  strengthened  by  a  comparison  with  that  which 
produces  the  August  meteors,  where  we  lind  a  decided  inark 
of  antiquity.  Here  the  swiftest  of  tlie  group  has,  in  the  course 
of  numerous  revohitions,  overtaken  the  slowest,  so  that  the 
gronp  is  now  spread  almost  equally  around  the  entire  orbit. 
The  time  of  revolution  being,  in  this  case,  more  than  a  cen- 
tury, this  equal  distribution  would  take  a  much  longer  time 
than  in  the  other  case,  where  the  period  is  only  thirty-three 
years;  so  that  we  can  say,  with  considerable  probability,  that 
the  August  group  has  been  in  our  system  at  least  twenty 
times  as  long  as  the  November  group. 

§  7.  The  Physical  Constitution  of  Comets. 

A  theory  of  the  physical  constitution  of  comets,  to  be  both 
complete  and  satisfactory,  must  be  founded  on  the  properties 
of  matter  as  made  known  to  ns  here  at  the  surface  of  the 
earth.  That  is,  we  must  show  what  forms  and  what  combina- 
tions of  known  substances  would,  if  projected  into  the  celes- 
tial spaces,  present  the  appearance  of  a  comet.  Now,  this  has 
never  yet  been  completely  done.  Theories  without  number 
have  been  propounded,  but  they  fail  to  explain  some  of  the 
phenomena,  or  explain  them  in  a  manner  not  consistent  with 
the  known  laws  of  matter  or  force.  We  cannot  stop  even  to 
mention  most  of  these  theories,  and  shall  therefore  confine  our 
attention  to  those  propositions  which  are  to  some  extent  sus- 
tained by  facts,  and  which,  on  the  whole,  seem  to  have  most 
probability  in  their  favor. 

The  simplest  form  of  these  bodies  is  seen  in  the  telescopic 
comets,  which  consist  of  minute  particles  of  a  cloudy  or  vapor- 
ous appearance.  Now,  we  know  that  masses  which  present 
this  appearance  at  the  surface  of  the  earth,  where  we  can  ex- 
amine them,  are  composed  of  detached  particles  of  solid  or 
liquid  matter.  Clouds  and  vapor,  for  instance,  are  composed 
of  minute  drops  of  water,  and  smoke  of  very  minute  particles 
of  carbon.  Analogy  would  lead  us  to  suppose  that  the  tele- 
scopic comets  are  of  the  same  constitution.     They  are  gener- 


THE  PHYSICAL  CONSTITUTION  OF  COMETS.  399 

all}'  tens  of  thousands  of  miles  in  diameter,  and  yet  of  snch 
tenuity  that  the  smallest  stars  are  seen  thronj^h  them.  The 
strongest  evidence  of  this  constitution  is,  however,  aift>rded  by 
the  phenomena  of  meteoric  showers  described  in  the  last  sec- 
tion. We  have  seen  that  these  are  caused  by  our  atmosphere 
encountering  the  debris  of  comets,  and  this  debris  presents  it- 
self in  the  form  of  detached  meteoroids,  of  very  small  magni- 
tude, but  hundreds  of  miles  apart. 

The  only  alternative  to  this  theory  is  that  the  comet  is  a 
mass  of  true  gas,  continuous  throughout  its  whole  extent. 
This  gaseous  theory  derives  its  main  support  from  tlie  spec- 
troscope, which  shows  the  spectrum  of  the  telescopic  comets 
to  consist  of  bright  bands,  the  mark  of  an  incandescent  gas. 
Moreover,  the  resemblance  of  these  bands  to  those  produced 
by  the  vapor  of  carbon  is  so  striking  that  it  is  quite  common 
among  spectroscopists  to  speak  of  a  comet  as  consisting  of 
the  gas  of  some  of  the  compounds  of  carbon.  But  tliei-e  are 
several  difficulties  which  look  insuperable  in  the  way  of  the 
theory  that  a  comet  is  nothing  but  a  mass  of  gas.  In  the 
first  place,  the  elastic  force  of  such  a  mass  would  cause  it 
to  expand  beyond  all  limits  when  placed  in  a  position  where 
there  is  absolutely  no  pressure  to  confine  it,  as  in  the  celestial 
spaces.  Again,  a  gas  cannot,  so  far  as  experiment  has  ever 
gone,  shine  by  its  own  light  until  it  is  heated  to  a  high  tem- 
perature, far  aliove  any  that  can  possibly  exist  at  distances 
from  tlie  sun  so  great  as  those  at  which  comets  have  been 
situated  when  under  examination  with  the  spectroscope.  Fi- 
nally, in  the  event  of  a  purely  gaseous  comet  being  broken 
up  and  dissipated,  as  in  the  case  of  Biela's  comet,  it  is  hardly 
possible  to  suppose  that  it  would  separate  into  innumerable 
widely  detached  pieces,  as  this  comet  did.  The  gaseous  the- 
ory can,  therefore,  not  be  regarded  as  satisfactory.  It  may  be 
that  comets  will  hereafter  be  found  to  consist  of  some  combi- 
nation of  solid  and  caseous  matter,  the  exact  nature  of  which 
is  not  yet  determined  ;  or  it  may  be  that  this  matter  is  of  a 
nature  or  in  a  form  wholly  unlike  anything  that  we  are  ac- 
quainted with  or  can  produce  here  on  the  earth.     As  tlie  case 


400  THE  SOLAR  SYSTEM.   . 

now  stands,  we  must  rc<ji;ard  the  spectnini  of  a  comet  as  some- 
thing not  jet  satisfactorily  accounted  for. 

When  we  turn  from  telescoj)ic  comets  to  those  brilliant 
ones  which  exhibit  a  nucleus  and  a  tail,  we  can  trace  certain 
operations  which  are  not  seen  in  the  case  of  the  others.  What 
the  nucleus  is — whether  it  is  a  solid  bodv  several  hundred  miles 
in  diameter,  or  a  dense  mass  of  the  same  materials  which  com- 
pose a  telescopic  comet — we  are  quite  unable  to  say.  But 
there  can  hardly  be  any  reasonable  doubt  that  it  is  composed 
of  some  substance  which  is  vaporized  by  the  heat  of  the  solar 
rays.  The  head  of  such  a  comet,  when  carefully  examined 
with  the  telescope,  is  found  to  be  composed  of  successive  en- 
velopes or  layers  of  \apor ;  and  when  these  envelopes  are 
watched  from  night  to  night,  they  are  found  to  be  gradually 
rising  upwards,  growing  fainter  and  more  indistinct  in  out- 
line as  they  attain  a  greater  elevation,  until  they  are  lost  in 
the  outlying  parts  of  the  coma.  These  rising  masses  form  the 
fan-shaped  appendage  described  in  a  preceding  section. 

The  strongest  proof  that  some  evaporating  process  is  going 
on  from  the  nucleus  of  the  comet  is  afforded  by  the  move- 
ments of  the  tail.  It  has  long  been  evident  that  the  tail  could 
not  be  an  appendage  which  the  comet  carried  along  with  it, 
and  this  for  two  reasons:  first,  it  is  impossible  that  there  could 
be  any  cohesion  in  a  mass  of  matter  of  such  tenuity  that  the 
smallest  stars  could  be  seen  through  a  million  of  miles  of  it, 
and  which,  besides,  constantly  changes  its  form ;  secondly,  as 
a  comet  flies  around  the  sun  in  its  immediate  neighborhood, 
the  tail  a})pears  to  move  from  one  side  of  the  sun  to  another 
with  a  rapidity  which  would  tear  it  to  pieces,  and  send  the 
separate  parts  flying  off  in  hyperbolic  orbits,  if  the  movement 
were  real.  The  inevitable  conclusion  is  that  the  tail  is  not  a 
fixed  appendage  of  the  comet,  which  the  latter  carries  with  it, 
but  a  stream  of  vapor  rising  from  it,  like  smoke  from  a  chim- 
ney. As  the  line  of  smoke  which  \ve  now  see  coming  from 
the  chimney  is  not  the  same  M'hich  we  saw  a  minute  ago,  be- 
cause the  latter  has  been  blown  away  and  dissipated,  so  we  do 
not  see  the  satne  tail  of  a  comet  all  the  time,  because  the  mat- 


THE  PEY€ICAL  CONSTITVTIOX  OF  COMETS.  401 

ter  which  makes  up  the  tail  is  constantly  streaming  outwards, 
and  constantly  being  replaced  by  new  vapor  rising  from  the 
nucleus.  The  evaporation  is,  no  doubt,  due  to  the  heat  of  the 
sun,  for  there  can  be  no  evaporation  without  her^i:,  and  the 
tails  of  comets  increase  enormously  as  they  approach  the  sun. 
Altogether,  a  good  idea  of  the  operations  going  on  in  a  comet 
will  be  obtained  if  we  conceive  the  nucleus  to  be  composed  of 
water  or  other  volatile  fluid  which  is  boiling  away  under  the 
heat  of  the  sun,  while  the  tail  is  a  colunm  of  steam  rising 
from  it. 

We  now  meet  a  question  to  which  science  has  not  yet  been 
able  to  return  a  conclusive  answer.  Why  does  this  mass  of 
vapor  always  fly  away  from  the  sun  ?  That  the  matter  of  the 
comet  should  be  vaporized  by  the  sun's  rays,  and  that  the  nu- 
cleus should  thus  be  enveloped  in  a  cloud  of  vapor,  is  perfect- 
ly natural,  and  entirely  in  accord  with  the  properties  of  mat- 
ter which  we  observe  around  us.  l^ut,  according  to  all  known 
laws  of  matter,  this  vapor  should  remain  around  the  head,  ex- 
cept that  the  outer  portions  would  be  gradually  detached  and 
thrown  off  into  separate  orbits.  There  is  no  known  tendency 
of  vapor,  as  seen  on  the  earth,  to  recede  from  the  sun,  and  no 
known  reason  why  it  should  so  recede  in  the  celestial  spaces. 
Various  theories  have  been  propounded  to  account  for  it ;  but 
as  they  do  not  rest  on  causes  which  we  have  verified  in  other 
cases,  they  must  be  regarded  as  purely  hypothetical. 

The  first  of  these  explanations,  in  the  order  of  time,  is  due 
to  Kepler,  who  conceived  the  matter  of  the  tail  to  be  driven 
off  by  the  impulsion  of  the  solar  rays,  which  thus  bleached 
the  comet  as  they  bleach  cloths  here.  If  light  were  an  emis- 
sion of  material  particles,  as  Newton  supposed  it  to  be,  this 
view  would  have  some  plausibility.  But  light  is  now  con- 
ceived to  consist  of  vibrations  in  an  ethereal  medium  ;  and 
there  is  no  kno^vn  way  in  which  they  could  exert  any  propel- 
ling force  on  matter.  Two  or  three  years  ago,  it  was  for 
a  while  supposed  that  the  "  radiometer  "  of  Mr.  Crookes  might 
really  indicate  such  an  action  of  the  solar  rays  upon  matter 
in  a  vacuum,  but  it  is  now  found  that  the  action  exhibited  is 

27 


402  THE  SOL  Alt  SYSTEM. 

really  due  to  a  minute  quantity  of  air  left  in  the  instrument. 
Had  Mr.  Crookes  shown  that  the  motion  of  his  radiometer 
was  really  due  to  the  impulsion  of  the  solar  rays,  we  might 
be  led  to  the  remarkable  conclusion  that  Kejjler's  theory, 
though  rejected  for  more  than  two  centuries,  was,  after  all, 
quite  near  the  truth. 

Sir  Isaac  Newton,  being  the  author  of  the  emission  theory 
of  light,  could  not  dispute  the  possibility  of  Kepler's  views 
being  correct,  but  nevertheless  gave  the  preference  to  anoth- 
er hyjiothesis.  He  conceived  the  celestial  spaces  to  be  filled 
with  a  very  rare  medium,  through  which  the  sun's  rays  passed 
without  heating  it,  as  they  pass  through  cold  air.  But  the 
comet  being  warmed  up  by  the  rays,  the  medium  surrounding 
it  is  warmed  up  by  contact,  and  thus  a  warm  current  is  sent 
out  from  the  comet,  just  as  a  current  of  warm  air  rises  from 
a  heated  body  on  the  surface  of  the  earth.  This  current  car- 
ries the  vapor  of  the  comet  with  it,  and  thus  gives  rise  to  the 
tail  in  the  same  way  that  the  current  of  warm  air  rising  from 
a  chimney  carries  up  a  column  of  smoke.  It  has  long  been 
established  that  there  is  no  medium  in  the  planetary  spaces 
in  which  such  an  effect  as  this  is  possible :  Newton's  theory 
is,  therefore,  no  longer  considered.  ; 

In  recent  times,  Zollner  has  endeavored  to  account  for  the 
tail  of  the  comet  by  an  electrical  action  between  the  sun  and 
the  vapor  rising  from  the  nucleus  of  the  comet.  The  various 
papers  in  which  he  has  elaborated  his  views  of  the  constitu- 
tion of  comets  are  marked  by  profound  research  ;  and  we 
must  regard  his  theories  as  those  which,  on  the  whole,  most 
completely  explain  all  the  phenomena.  But  they  still  lack 
the  one  thing  needful  to  secure  their  reception :  there  is  no 
evidence  that  the  sun  acts  as  an  electrified  body ;  and  until 
such  evidence  is  adduced  by  experiment,  or  by  observation  on 
other  bodies  than  comets,  the  electric  theory  of  the  comet's 
tail  can  only  be  regarded  as  a  more  or  less  probable  hypothe- 
sis. Indeed,  some  physicists  claim  that  any  such  electric  ac- 
tion in  the  planetary  spaces  is  impossible.  Before  any  theory 
can  be  definitely  settled  upon,  accurate  observations  must  be 


THE  rUTSICAL  CONSTITVTIOX  OF  COMETH.  403 

made  upon  the  tails  of  comets  witli  a  view  of  learning  the 
law  according  to  which  the  vapor  is  repelled  from  the  sun, 
Sucli  observations  were  made  by  Bcssel  on  llalley's  comet  in 
1835,  and  by  various  observers  on  the  great  comet  of  1858. 
The  former  were  investigated  by  I'essel  himself,  and  the  lat- 
ter by  several  niathenuiticians,  among  them  Professor  Peirce, 
whose  results  are  found  in  a  paper  connnunicated  to  the 
American  Academy  in  1859.  lie  found  the  repulsive  force 
of  the  sun  upon  the  particles  which  form  the  front  edge  of 
the  tail  to  be  1^^  times  its  attractive  force  upon  ordinary 
bodies  at  the  same  distance.  It  seemed  constantly  to  diminish 
as  the  back  edge  of  the  tail  was  approached ;  but,  owing  to 
the  poor  definition  of  this  edge,  and  the  uncertainty  whether  it 
was  composed  of  a  continuous  stream  of  particles,  the  amount 
of  the  diminution  could  not  be  accurately  lixed.  The  suc- 
cessive envelopes  were  found  to  ascend  uniformly  towards 
the  sun  at  the  rate  of  about  thirty-five  miles  an  hour.  Bond, 
from  a  careful  examination  of  all  the  observations,  M-as  led  to 
the  result  that  the  rate  of  ascent  diminished  as  the  height 
became  greater. 

An  apparently  necessary  conclusion  from  this  constant  evap- 
oration and  expulsion  of  vapor  from  comets  with  tails  is,  that 
such  bodies  are  constantly  wasting  away  when  in  the  neigh- 
borhood of  the  sun.  This  conclusion  is  strengthened  by  the 
fact  that  not  a  single  comet  of  very  short  period  has  a  consid- 
erable tail,  the  probability  being  that  all  the  volatile  matter 
^vhich  once  went  to  form  the  tail  has  been  eva])orated.  In- 
deed, from  the  descriptions  of  the  old  chroniclers,  it  has  been 
supposed  that  llalley's  comet  had  a  nnich  more  conspicuous 
tail  at  the  time  of  its  earliest  recorded  apparitions  than  it  has 
exhibited  at  its  last  few  returns.  There  is,  however,  no  neces- 
sity for  supposing  the  diminution  so  rapid  as  this,  for  the 
amount  of  matter  really  necessary  to  make  the  most  splendid 
tail  is  so  extremely  small  that  a  comet  might  lose  it  a  hundred 
times  over  without  becoming  perceptibly  smaller.  This  con- 
stant loss  of  matter  through  the  tail  affords  an  additional 
ground  for  the  view  that  comets  in  general  are  visitors  Intro- 


404  THE  SOLAR  SYSTEM. 

duced  into  onr  system  by  the  action  of  the  planets.  If,  for 
instance,  such  a  comet  as  Ilalley's  had  been  a  member  of  our 
system  for  millions  of  years,  and  had  returned  to  perihelion  a 
hundred  thousand  times,  all  its  volatile  matter  must  long  ago 
have  evaporated. 

The  question  of  the  mass  and  density  of  comets  is  also  one 
of  those  on  which  it  is  difficult  to  reach  satisfactory  conclu- 
sions. We  cannot  certainly  decide  from  mere  telescopic  ob- 
servation whether  the  nucleus  is  a  single  large  body,  like  a 
planet  or  satellite,  or  whether  it  is  merely  the  densest  part  of 
an  immense  cloud  of  meteoroids.  The  mass  of  nebulous  mat- 
ter which  surrounds  the  nucleus  increases  so  gradually  as  we 
approach  the  central  parts,  that  it  is  hardly  possible  to  decide 
where  the  nucleus  begins:  the  more  powerful  the  telescope, 
the  smaller  the  nucleus  generally  appears.  Moreover,  in  the 
same  comet,  the  apparent  magnitude  of  tlie  nucleus  is  subject 
to  immense  variations,  thus  showing  that  it  cannot  be  a  solid 
body  out  to  its  apparent  limits.  If  we  considered  only  this 
circumstance,  and  the  general  analogy  with  telescopic  comets, 
we  should  say  that  even  the  densest  part  of  the  comet  was 
i.jthing  but  a  cloud  of  solid  or  liquid  particles  so  thick  that  it 
looked  solid,  as  a  cloud  does  in  our  sky.  But  if  this  was  the 
case,  as  Professor  Peirce  showed  in  his  investigations  of  the 
comet  of  1858,  the  comets  of  1680  and  of  1843  must  have 
been  completely  pulled  apart  by  the  enormous  tidal  forces 
generated  by  their  near  approach  to  the  sun.  In  the  opinion 
of  this  investigator,  the  fact  that  they  went  through  such  an 
ordeal  shows  them  to  be  of  metallic  density. 

The  question  is  frequently  asked,  What  would  be  the  effect 
if  a  comet  should  strike  the  earth  ?  This  would  depend  upon 
what  sort  of  a  comet  it  was,  and  what  part  of  the  comet  came 
in  contact  with  our  planet.  The  latter  might  pass  through 
the  tail  of  the  largest  comet  without  the  slightest  effect  being 
produced,  the  tail  being  so  thin  and  airy  that  a  million  miles 
thickness  of  it  looks  only  like  gauze  in  the  sunlight.  It  is 
not  at  all  unlikely  that  such  a  thing  may  have  happened  with- 
out ever  being  noticed.    A  passage  through  a  telescopic  comet 


THE  ZODIACAL  LIGHT.  405 

would  be  accompanied  by  a  ])rilliant  meteoric  shower,  prob- 
ably a  far  more  brilliant  one  than  has  ever  been  recorded. 
No  more  serious  danger  would  be  encountered  than  that  aris- 
ing from  a  possible  fall  of  meteorites.  But  a  collision  between 
the  nucleus  of  a  large  comet  and  the  earth  might  be  a  serious 
matter.  If,  as  Professor  Peirce  8up])oses,  the  nucleus  is  a  solid 
body  of  metallic  density,  many  miles  in  diameter,  the  effect 
where  the  comet  struck  would  be  terrific  beyond  conception. 
At  the  first  contact  in  the  upper  regions  of  the  atmosphere, 
the  whole  heavens  would  be  illuminated  with  a  resplendence 
beyond  that  of  a  thousand  suns,  the  sky  radiating  a  light  which 
would  blind  every  eye  that  beheld  it,  and  a  heat  which  would 
melt  the  hardest  rocks.  A  few  seconds  of  this,  while  the  huge 
body  was  passing  through  the  atmosphere,  and  the  collision  at 
the  earth's  surface  would  in  an  instant  reduce  everything  there 
existing  to  fiery  vapor,  and  bury  it  miles  deep  in  the  solid 
earth.  Happily,  the  chances  of  such  a  calamity  are  so  minute 
that  they  need  not  cause  the  slightest  uneasiness.  There  is 
hardly  a  possible  form  of  death  which  is  not  a  thousand  times 
more  probable  than  this.  So  small  is  the  earth  in  comparison 
with  the  celestial  spaces,  that  if  one  should  shut  his  eyes  and 
fire  a  gun  at  random  in  the  air,  the  chance  of  bringing  down 
a  bird  would  be  better  than  that  of  a  comet  of  any  kind  strik- 
ing the  earth. 

§  8.  Tlie  Zodiacal  Lvjlit. 

This  object  consists  (»f  a  very  soft,  faint  column  of  light, 
which  may  be  seen  rising  from  the  western  horizon  tfter  twi- 
light on  any  clear  winter  or  spring  evening:  it  may  also  be 
seen  rising  from  the  eastern  horizon  just  before  daybreak  in 
the  summer  or  autumn.  It  really  extends  out  on  each  side 
of  the  sun,  and  lies  nearly  in  the  plane  of  the  ecliptic.  The 
reason  it  cannot  be  well  seen  in  the  summer  and  autumn 
evenings  is,  that  in  our  latitudes  the  course  of  the  ecliptic  in 
the  south-west  is,  during  those  seasons,  so  near  the  horizon  that 
the  light  in  question  is  extinguished  by  the  great  thickness  of 
atmosphere  through  which  it  has  to  pass.    Near  the  equator, 


406  THE  SOLAB  SYSTEM. 

where  the  ecliptic  always  rises  high  above  the  horizon,  the 
light  can  be  seen  about  equally  well  all  the  year  round.  It 
grows  fainter  the  farther  it  is  from  the  sun,  and  can  gener- 
ally be  traced  to  about  90°  from  that  luminary,  when  it  grad- 
ually fades  away.  But  in  a  very  clear  atmosphere,  between 
the  tropics,  it  has  been  traced  all  the  way  across  the  heavens, 
from  east  to  west,  thus  forming  a  complete  ring. 

Such  is  the  zodiacal  light  as  it  appears  to  the  eye.  Put- 
ting its  appearances  all  together,  we  may  see  that  it  is  due  to 
a  lens -shaped  appendage  of  some  sort  surrounding  the  sun, 
and  extending  out  a  little  beyond  the  earth's  orbit.  It  lies 
very  nearly  in  the  plane  of  the  ecliptic,  but  its  exact  position 
is  difficult  to  determine,  not  only  owing  to  its  indistinct  out- 
line, but  because  in  northern  latitudes  the  southern  edge  will 
be  dimmed  by  the  greater  thickness  of  atmosphere  through 
which  it  is  seen,  and  thus  the  light  will  look  farther  north 
than  it  really  is.  The  nature  of  the  substance  from  which 
this  light  emanates  is  entirely  unknown.  Its  spectrum  has 
been  examined  by  several  observers,  some  of  whom  have  re- 
ported it  as  consisting  of  a  single  yellow  line,  and  therefore 
arising  from  a!i  incandescent  gas.  This  would  indicate  a  len- 
ticular-shaped atmosphere  of  inconceivable  rarity  surrounding 
the  sun,  and  extending  out  near  the  i)lane  of  the  ecliptic  be- 
yond the  orbit  of  the  earth.  But  Professor  Wright,  of  Yale 
College,  who  has  made  the  most  careful  observations  of  this 
spectrum,  finds  it  to  be  continuous.  For  several  reasons,  too 
minute  to  enter  into  now,  this  observation  seems  to  the  Avriter 
more  likely  to  be  correct.  Accepting  it,  we  should  be  led  to 
the  conclusion  that  the  phenomenon  in  question  is  due  to  re- 
flected sunlight,  probably  from  an  immense  cloud  of  meteor- 
oids  filling  up  the  upace  between  the  earth  and  sun.  But  fur- 
ther researches  must  be  made  before  a  conclusive  result  can 
be  reached. 


PART  IV.— THE  STELLAR  UNIVERSE. 


INTRODUCTOEY  REMARKS. 

Hitherto  our  attention  has  been  principally  occupied  with 
the  bodies  which  surround  onr  sun  and  make  up  the  solar  sys- 
tem. Notwithstanding  the  immense  distances  at  which  these 
bodies  are  found,  we  may  regard  them,  in  comparison  with  the 
fixed  stars,  as  an  isolated  family  immediately  surrounding  us, 
since  a  sphere  as  large  as  the  whole  solar  system  would  only 
appear  as  a  point  to  the  vision  if  viewed  from  the  nearest 
star.  The  space  which  separates  the  orbit  of  Neptune  from 
the  fixed  stars  and  the  fixed  stars  from  eacli  other  is,  so  far  as 
we  can  learn,  entirely  void  of  all  visible  matter,  except  occa- 
sional waste  nebulous  fragments  of  a  meteoric  or  cometary 
nature  which  are  now  and  then  drawn  in  by  the  attraction  of 
our  sun. 

The  widest  question  which  the  study  of  the  stars  presents 
to  us  may  be  approached  in  this  way :  We  have  seen,  in  our 
system  of  sun,  i)lanets,  and  satellites,  a  very  orderly  and 
beautiful  structure,  every  body  being  kept  in  its  own  orbit 
through  endless  revolutions  by  a  constant  balancing  of  gravi- 
tating and  centrifugal  forces.  Do  the  millions  of  suns  and 
clusters  scattered  through  space,  and  brought  into  view  by  the 
telescope,  constitute  a  greater  S3'stem  of  equally  orderly  struct- 
ure? and,  if  so,  what  is  that  structure?  If  we  measure  the 
importance  of  a  question,  not  by  its  relations  to  our  interests 
and  our  welfare,  but  by  the  intrinsic  greatness  of  the  subject 
to  which  it  relates,  then  we  must  regard  this  question  as  one 
of  the  noblest  with  which  the  human  mind  has  ever  been 


408  THE  STELLAR   UNIVERSE. 

occupied.  In  piercing  the  mystery  of  the  solar  system,  and 
showing  that  the  earth  on  which  we  dwell  was  only  one  of 
the  smaller  of  eight  planets  which  move  around  the  sun,  we 
made  a  great  step  in  the  way  of  enlarging  our  ideas  of  the 
immensity  of  creation  and  of  the  comparative  insignificance 
of  our  sublunary  interests.  But  when,  on  extending  our  view, 
we  find  our  sun  to  be  but  one  out  of  unnumbered  millions,  M'e 
see  that  our  whole  system  is  but  an  insignificant  part  of  crea- 
tion, and  that  we  have  an  immensely  greater  fabric  to  study. 
When  we  have  bound  all  the  stars,  nebulae,  and  clusters  which 
our  telescopes  reveal  into  a  single  system,  and  shown  in  what 
manner  each  stands  related  to  all  the  others,  we  shall  have 
solved  the  problem  of  the  material  universe,  considered,  not  in 
its  details,  but  in  its  widest  scope. 

From  the  time  that  Copernicus  showed  the  stars  to  be  self- 
luminous  bodies,  situated  far  outside  of  our  solar  system,  the 
question  thus  presented  has  occupied  the  attention  of  the  pliil- 
osophical  class  of  astronomers.  The  original  view,  which  has 
been  the  starting-point  of  all  speculation  on  the  subject,  we 
have  described  in  the  Introduction  as  that  of  a  spherical  uni- 
verse. The  apparent  sphericity  of  the  vault  of  heaven,  the 
uniformity  of  the  diurnal  revolution,  and  the  invariability  of 
the  relative  positions  of  the  stars,  all  combined  to  strengthen 
the  idea  that  the  latter  were  set  on  the  interior  surface  of  a 
hollow  sphere,  having  the  earth  or  the  sun  in  its  centre.  This 
sphere  constituted  the  firmament  of  the  ancients,  outside  of 
which  was  situated  the  empyrean,  or  kingdom  of  fire.  Coper- 
nicus made  no  advance  whatever  on  this  idea.  Galileo  and 
Kepler  seem  to  have  made  the  fii'st  real  advance — the  former 
by  resolving  the  Milky  Way  into  stars  with  his  telescope,  the 
latter  by  suggesting  that  our  sun  might  be  simply  one  of  nu- 
merous stars  scattered  through  space,  looking  so  bright  only 
on  account  of  our  proximity  to  it.  In  the  problem  of  the 
stellar  system  this  conception  held  the  same  important  place 
which  that  of  the  earth  as  a  ])lanet  did  in  the  problem  of  the 
solar  system.  But  Kepler  was  less  fortunate  than  Copernicus 
in  that  he  failed  to  commend  his  idea,  even  to  his  own  judg- 


INTRODUCTOEY  REMARKS.  409 

ment.  It  was  by  affording  a  starting-point  for  the  researches 
of  Kant  and  Herschel  that  Kepler's  suggestion  really  bore 
fruit. 

Notwithstanding  the  amount  of  careful  research  which 
Herschel  and  his  successors  have  devoted  to  it,  we  are  still 
very  far  from  having  reached  even  an  approximate  solution 
of  the  problem  of  which  we  speak.  In  w^hatever  direction  we 
pursue  it,  we  soon  find  ourselves  brought  face  to  face  with  the 
infinite  in  space  and  time.  Especially  is  this  the  case  when 
we  seek  to  know,  not  simply  what  the  universe  is  to-day,  but 
what  causes  are  modifying  it  from  age  to  age.  All  the  knowl- 
edge that  man  has  yet  gathered  is  then  found  to  amount  to 
nothing  but  some  faint  glimmers  of  light  shining  here  and 
tliere  through  the  seemingly  boundless  darkness.  The  glim- 
mer is  a  little  brighter  for  each  successive  generation,  but 
many  centuries  must  elapse  before  we  can  do  much  more 
than  tell  how  the  nearer  stars  are  situated  in  space.  Indeed, 
we  see  as  yet  but  little  hope  that  an  inhabitant  of  this  planet 
will  ever,  from  his  own  observations  and  those  of  his  prede- 
cessors, be  able  to  completely  penetrate  the  mystery  in  which 
the  structure  and  destiny  of  the  cosmos  are  now  enshrouded. 
However  this  may  be  in  the  future,  all  we  can  do  at  present 
is  to  form  more  or  less  probable  conjectures,  founded  on  all 
we  know  of  the  general  character  of  natural  law.  In  a  strictly 
scientific  treatise,  such  conjectures  would  find  no  place  ;  and 
if  we  had  to  grope  in  absolute  darkness,  they  would  be  en- 
tirely inappropriate  in  any  but  a  poetical  or  religious  produc- 
tion. But  the  subject  is  too  fascinating  to  permit  us  to  neg- 
lect the  faintest  light  by  the  aid  of  which  we  may  penetrate 
the  mystery;  we  shall  therefore  briefly  set  forth  both  what 
men  of  the  past  have  thought  on  the  subject,  what  the  science 
of  to  day  enables  us  to  assert  with  some  degree  of  probability, 
and  what  knowledge  it  wholly  denies  us.  To  proceed  in  sci- 
entific order,  we  must  commence  by  laying  a  wide  foundation 
of  facts.  Our  first  step  will  therefore  be  to  describe  the  heav- 
ens as  they  appear  to  the  naked  eye,  and  as  they  are  seen  in 
the  telescope. 


410  THE  STELLAR   UNIVERSE. 


CHAPTER  I. 

THE    STARS   AS   THEY    ARE    SEEN. 

§  1.  Number  and  Orders  of  Stars  and  Nebidce. 

The  total  number  of  stare  in  the  celestial  spliere  visible 
with  the  average  naked  eye  may  be  estimated,  in  round  num- 
bers, as  5000.  The  number  varies  so  much  with  the  perfec- 
tion and  training  of  the  eye,  and  with  the  atmospheric  condi- 
tions, that  it  camiot  be  stated  very  definitely.  When  the  tele- 
scope is  pointed  at  the  heavens,  it  is  found  that  for  every  star 
visible  to  the  naked  eye  there  are  hundreds,  or  even  thousands, 
too  minute  to  be  seen  without  artificial  aid.  From  the  counts 
of  stars  made  by  llerschel,  Struve  has  estimated  that  the  total 
number  of  stars  visible  with  llerschel's  twenty-foot  telescope 
was  about  20,000,000.  The  great  telescopes  of  modern  times 
would,  no  doubt,  show  a  yet  larger  number ;  but  a  reliable 
estimate  has  not  been  made.  The  number  is  probably  some- 
where between  30,000,000  and  50,000,000. 

At  a  very  early  age,  the  stars  were  classified  according  to 
their  apparent  brightness  or  magnitude.  The  fifteen  brightest 
ones  were  said  to  be  of  the  first  magnitude;  the  fifty  next  in 
order  were  termed  of  the  second  magnitude,  and  so  on  to  the 
sixth,  which  comprised  the  faintest  stars  visible  to  the  naked 
eye.  The  number  of  stars  of  each  order  of  magnitude  be- 
tween the  north  pole  and  the  circle  35°  south  of  the  equator 
is  about  as  follows : 

Of  mngnitudo  1  there  are  about 14  stars. 

"  2  "  48     " 

••  8  ♦♦  152     " 

«'  4  "  3i;$     " 

«•  6  "  S'A     " 

"  6  "  2010     " 

Total  visible  to  naked  eve 33;)1     " 


NUMBER  AND   ORDERS  OF  STARS  AND  NEBULA.       411 

This  limit  includes  all  tlio  stars  which,  in  the  Middle  States, 
culminate  at  a  greater  altitude  than  15°.  The  number  of  the 
sixth  magnitude  which  can  be  seen  dejiends  very  nnich  upon 
the  eye  of  the  observer  and  the  state  of  the  sky.  The  foi-ego- 
ing  list  includes  all  that  can  be  seen  by  an  ordinary  good  eye 
in  a  clear  sky  when  there  is  no  moonlight ;  but  the  German 
astronomer  Heis,  from  whom  these  numbers  are  taken,  gives  a 
list  of  1964  more  which  he  believes  he  can  see  without  a  glass. 

'The  system  of  expressing  the  brightness  of  the  stars  by  a 
series  of  numbers  is  continued  to  the  telescopic  stars.  The 
smallest  star  visible  with  a  six-inch  telescope  under  ordinary 
circumstances  is  conunonly  rated  as  of  the  thirteenth  magni- 
tude. On  the  same  scale,  the  stnallest  stars  visible  with  the 
largest  telescopes  of  the  world  would  be  of  about  the  six- 
teenth magnitude,  but  no  exact  scale  for  these  very  faint  stars 
has  been  arranged. 

Measures  of  the  relative  brilliancy  of  the  stars  indicate 
that,  as  we  descend  in  the  scale  of  magnitude,  the  quantity 
of  light  emitted  diminishes  in  a  geometrical  ratio,  the  stars 
of  each  order  being,  in  general,  between  two-iifths  and  one- 
third  as  bright  as  those  of  the  order  next  above  them.  This 
order  of  diminution  is  not,  however,  exact,  because  the  arrange- 
ment of  magnitudes  has  been  made  by  mere  estimation  of  in- 
dividual observei's  who  may  have  hit  on  different  and  varying 
ratios;  hut  it  is^a  sufficient  approach  to  th*:  truth  for  fonnnon 
purposes.  From  the  second  to  tlie  fifth  magnitude  the  dimi- 
nution is  probably  one -third  in  each  magnitude,  after  that 
about  two-fifths.  Supposing  the  ratio  two-iifths  to  be  exact, 
we  find  that  it  would  take  al)out 

2^  stars  of  tlie  second  magnitude  to  make  one  of  the  first. 


6 

tliird 

16 

fourth 

40 

fifth 

100 

sixth 

10,000 

eleventh 

1,000,000 

sixteenth 

Tiie  number 

of 

stars  of  the 

sevei 

•al 

scales 

of 

magnitude 

varv  in  a  I'atio  not  far  different  from  the  inverse  of  that  of 


412  THE  STELLAR  UNIVERSE. 

their  brightness,  the  ratio  being  a  little  greater  in  the  case  of 
the  higher  magnitudes,  and  probably  a  little  less  in  the  case 
of  the  lower  ones.  Thus,  we  see  that  there  are  about  three 
times  as  many  stars  of  the  second  magnitude  as  of  the  first, 
three  times  as  many  of  the  third  as  of  the  second,  and  after 
that  something  less  than  three  times  as  many  of  each  magni- 
tude as  of  the  magnitude  next  above.  Comparing  this  with 
the  table  of  relative  briglitness  just  given,  we  may  conclude 
that  if  all  the  stars  of  each  magnitude  were  condensed  into  a 
single  one,  the  brightness  of  the  combined  stars  thus  formed 
would  not  vary  extravagantly  from  one  to  another  until  we 
had  passed  beyond  tlie  ninth  or  tenth  magnitude.  But  it  is 
certain  that  the  brightness  would  ultimately  diminish,  because 
otherwise  there  would  be  no  limit  to  the  total  amount  of  light 
given  by  the  stars,  and  the  whole  heavens  would  shine  like 
the  sun. 

Tlie  reader  will,  of  course,  understand  that  this  arrange- 
ment by  magnitude  is  purely  artificial.  Really  the  stars  are 
of  every  order  of  brightness,  varying  by  gradations  which  are 
entirely  insensible,  so  that  it  is  impossible  to  distinguish  be- 
tween the  briglitest  star  of  one  magnitude  and  the  faintest  of 
the  magnitude  next  above  it.  Hence,  those  astronomers  who 
wish  to  express  magnitudes  with  the  greatest  exactness,  divide 
them  into  thirds  or  even  tenths ;  so  that,  for  instance,  stars  be- 
tween the  sixth  and  seventh  magnitudes  are  called  6.1,  6.2, 
6.3,  and  so  on  to  6.9,  according  to  their  brilliancy.  Various 
attempts  have  been  made  to  place  the  problem  of  the  relative 
amounts  of  light  emitted  by  the  stars  upon  a  more  exact  basis 
than  this  old  one  of  magnitudes,  but  this  is  a  very  difficult 
thing  to  do,  because  there  is  no  way  of  measuring  light  except 
by  estimation  with  the  eye.  In  order  to  measure  the  relative 
intensity  of  two  lights,  it  is  necessary  to  have  some  instrument 
by  wliich  the  intensity  of  one  or  both  the  lights  ma}'  be  varied 
until  the  two  appear  to  be  equal.  Instruments  for  this  pur- 
pose are  known  as  photometers,  and  are  of  various  construc- 
tions. For  comparing  the  liglit  of  different  stars,  the  photom- 
eter most  used  at  tlie  present  time  is  that  of  Ztillner.     By 


NUMBER  AND   ORDERS  OF  STARS  AND  NEBULJi,      413 

this  instrument  the  light  of  the  stars,  as  seen  through  a  small 
telescope,  is  compared  both  in  color  and  intensity  with  that  of 
an  artificial  star,  the  liglit  of  which  can  be  varied  at  pleasure. 
A  complete  set  of  measures  with  this  instrument,  including 
most  of  the  brighter  stars,  is  one  of  tlie  wants  of  astronomy 
which  we  may  soon  hope  to  see  supplied.  The  most  extended 
recent  series  of  photometric  estimates  with  which  the  writer 
is  acquainted  is  that  of  Professor  Seidel,  of  Munich,  which  in- 
cludes 209  stars,  tlie  smallest  of  which  are  of  the  fifth  magni- 
tude. An  interesting  result  of  these  estimates  is  that  Sirius 
gives  us  four  times  as  much  light  as  any  other  star  visible  in 
our  latitude. 

Catalogues  of  Stars. — In  nearly  every  age  in  which  astron- 
omy has  llourislied  catalogues  of  stars  have  been  made,  giving 
their  positions  in  the  heavens,  and  the  magnitude  of  each. 
The  earliest  catalogue  which  has  come  to  us  is  found  in  the 
"Almagest"  of  Ptolemy,  and  is  supposed  to  be  that  of  Ilippar- 
chus,  who  flourished  150  years  before  the  Christian  era.  It 
is  said,  but  not  on  the  best  authority,  that  he  constructed  it  in 
order  that  future  generations  might  And  whether  any  change 
had  in  the  mean  time  taken  place  in  the  starry  heavens.  An 
examination  of  the  catalogue  shows  that  the  constellations  pre- 
sented much  the  same  aspect  two  thousand  years  ago  that  they 
do  now.  There»are  two  or  three  stars  of  his  catalogue  which 
cannot  now  be  certainly  identified ;  but  it  is  probable  tliat  the 
difficulty  arises  from  the  imperfection  of  the  catalogue,  and 
from  the  erroi*8  which  may  have  crept  into  the  numerous 
transcriptions  of  it  during  the  sixteen  centuries  whicli  elapsed 
before  the  art  of  printing  was  discovered.  The  catalogue  of 
Ilipparchus  contains  only  about  1080  stars,  so  that  he  could 
not  have  given  all  that  he  was  able  to  see.  lie  probably  omit- 
ted many  stars  of  the  smaller  magnitudes.  The  actual  num- 
ber given  in  the  "Almagest"  is  still  less,  being  only  1030. 

The  next  catalogue  in  the  order  of  time  is  that  of  Ulugh 
Beigh,  a  son  of  the  Tartar  monarch  Tamerlane,  which  dates 
from  the  fifteenth  century.  For  the  most  part,  the  stars  arc 
the  same  as  in  the  catalogue  of  Ptolemy,  only  the  places  were 


414  THE  STELLAR   UNIVERSE. 

redetermined  from  the  observations  at  Samarcand.  It  con- 
tains 1019  stars,  eleven  less  than  Ptolemy  gives.  Tjcho  Bralie, 
having  made  so  great  an  improvement  in  the  art  of  observa- 
tion, very  naturally  recatalogued  the  stars,  determining  their 
positions  with  yet  greater  accuracy  than  his  predecessors.  His 
catalogue  is  the  third  and  last  important  one  formed  before 
the  invention  of  the  telescope.     It  contains  1005  stars. 

Our  modern  catalogues  may  be  divided  into  two  classes: 
those  in  which  the  position  of  each  star  in  the  celestial  sphere 
(right  ascension  and  declination)  is  given  with  all  attainable 
precision,  and  those  in  which  it  is  only  given  approximately, 
so  as  to  identify  the  stai",  or  distinguish  it  from  others  in  its 
neighborhood.  The  catalogues  of  the  former  class  are  very 
numerous,  but  the  more  accurate  ones  are  necessarily  incom- 
plete, owing  to  the  great  labor  of  making  the  most  exact  de- 
termination of  the  position  of  a  star.  There  are,  perhaps, 
between  ten  or  twenty  thousand  stars  the  positions  of  which 
are  catalogued  with  astronomical  precision,  and  a  hundred 
thousand  more  in  which,  though  entire  precision  is  aimed  at, 
it  is  not  attained.  Of  the  merely  approximate  catalogues,  the 
greatest  one  is  the  "Sternverzeichniss"  of  Argelander,  which 
enumei-ates  all  the  stars  down  to  the  ninth  magnitude  between 
tlje  pole  and  two  degrees  south  of  the  equator.  The  work 
fills  three  thin  quarto  volumes,  and  the  entire  number  of  stars 
catalogued  in  it  exceeds  three  hundred  thousand.  This  "  star 
census"  is  being  continued  to  the  south  pole  at  the  observa- 
tory of  Cordoba,  South  America,  by  Dr.  Gould.  Of  the  mill- 
ions of  stars  of  the  tenth  magnitude  and  upwards,  hardly  one 
in  a  thousand  is,  or  can  be,  individually  known  or  catalogued. 
Except  as  one  or  another  may  exhibit  some  remarkable  pecu- 
liarity, they  must  pass  unnoticed  in  the  crowd. 

Division  into  Constellations. — A  single  glance  at  the  heavens 
shows  that  the  stars  are  not  equally  scattered  over  the  sky,  but 
that  great  numbers  of  them,  especially  of  the  brighter  ones, 
are  collected  into  extremely  irregular  groups,  known  as  con- 
stellations. At  a  very  early  age  the  heavens  were  represented 
as  painted  over  with  figures  of  men  and  animals,  so  arranged 


NUMBER  AND   ORDERS  OF  STARS  AND  NEBULA.      415 

as  to  include  the  principal  stars  of  each  constellation.  There 
is  no  historic  record  of  the  time  when  this  was  done,  nor  of  the 
principles  by  which  those  who  did  it  carried  out  their  v  n'k; 
but  many  of  the  names  indicate  that  it  was  during  the  I.oroic 
age.  Some  have  sought  to  connect  it  with  the  Argonautic  ex- 
pedition, from  the  fact  that  several  heroes  of  that  expedition 
were  among  those  thus  translated  to  the  heavens;  but  this  is 
little  more  than  conjecture.  So  little  pains  was  taken  to  fit 
the  figures  to  the  constelhitions  that  w^e  can  hardly  suppose 
them  to  have  all  been  executed  at  one  time,  or  on  anv  well- 
defined  plan.  Quite  likely,  in  the  case  of  names  of  heroes, 
the  original  object  was  rather  to  do  honor  to  the  man  than  to 
serve  any  useful  purpose  in  astronomy.  Whatever  their  ori- 
gin, these  names  have  been  retained  to  the  present  day,  al- 
though the  figures  which  they  originally  represented  no  longer 
serve  any  astronomical  purpose.  The  constellation  Hercules, 
for  instance,  still  exists ;  but  it  no  longer  represents  the  figure 
of  A  man  among  the  stars,  but  a  somewhat  irregular  portion 
of  the  heavens,  including  the  space  in  which  the  ancients 
placed  that  figure.  In  star-maps,  designed  for  school  instruc- 
tion and  for  common  use,  it  is  still  customary  to  give  these 
figures,  but  they  are  not  generally  found  on  maps  designed 
for  the  use  of  astronomers. 

XamuKj  the  /Skos. — The  question  how  to  name  the  individ- 
ual stars  in  each  constellation,  so  as  to  readily  distinguish 
them,  has  always  involved  some  difficulty.  In  the  ancient 
catalogues  they  were  distinguished  by  the  part  of  the  figure 
representing  the  constellation  in  which  they  were  found ;  as, 
the  eye  of  the  Bull,  the  tail  of  the  Great  Bear,  the  right  shoul- 
der of  Orion,  and  so  on.  The  Arabs  adopted  the  plan  of  giv- 
ing special  names  to  each  of  the  brighter  stars,  or  adopting 
such  names  from  the  Greeks.  Thus,  we  have  the  w^ell-known 
stars  Sirius,  Arcturus,  Procyon,  Aldebaran,  and  so  on.  Most 
of  these  names  have  dropped  entirely  out  of  astronomical  use, 
though  still  found  on  some  school  maps  of  the  stars.  The 
system  now  most  in  use  for  the  brighter  stars  was  designed  by 
Bayer,  of  Augsburg,  Germany,  about  1610.     lie  published  a 


416  THE  STELLAR   UNIVERSE. 

set  of  star-maps,  in  which  the  individual  stars  of  eacli  constel- 
lation were  designated  by  tlie  letters  of  tlie  Greek  alphabet — 
a,ft,y,  etc.  The  first  letters  were  given  to  the  brightest  stars, 
the  next  ones  to  the  next  brightest,  and  so  on.  After  the 
Greelv  letter  is  given  the  Latin  name  of  the  constellation  in 
the  genitive  case.  Thus,  Alpha  (o)  Scorpii,  or  Alpha  of  the 
Scorpion,  is  the  name  of  Arcturus,  the  brightest  star  in  Scor- 
pius ;  a  Lyra?,  of  the  brightest  star  in  the  Lyre ;  and  so  on. 
We  have  here  a  resemblance  to  our  system  of  naming  men, 
the  Greek  letter  corresponding  to  the  Christian  name,  and  the 
constellation  to  the  surname.  When  the  Greek  alphabet  was 
exhausted,  without  including  all  the  conspicuous  stars,  the 
Latin  alphabet  was  drawn  npon. 

The  Bayer  system  is  still  applied  to  all  the  stars  named  by 
him.  Most  of  the  other  stars  down  to  the  fifth  magnitude  are 
designated  by  a  system  of  numbers  assigned  by  Flamsteed  in 
his  catalogue.  Yet  other  stars  are  distinguished  by  their  num- 
bers in  some  well-known  catalogue.  When  this  method  fails, 
owing  to  the  star  not  being  catalogued,  the  position  in  the 
heavens  must  be  given. 

The  Milky  Wmj,  or  Galaxy. — To  the  naked  eye  so  much  of 
the  Galaxy  as  can  be  seen  at  one  time  presents  the  appearance 
of  a  white,  cloud-like  arch,  resting  on  two  opposite  points  of 
the  horizon,  and  rising  to  a  greater  or  less  altitude,  according 
to  the  position  of  the  celestial  sphere  relative  to  the  observer. 
Only  half  of  the  entire  arch  can  be  seen  above  the  horizon  at 
once,  the  other  half  being  below  it,  and  directly  opposite  the 
visible  half.  Indeed,  there  is  a  portion  of  it  which  can  never 
be  seen  in  our  latitude,  being  so  near  the  south  pole  that  it 
is  always  below  our  horizon.  If  the  earth  were  removed,  or 
made  transparent,  so  that  we  could  see  the  whole  celestial 
sphere  at  once,  the  Galaxy  would  appear  as  a  complete  belt 
extending  around  it.  The  telescope  shows  that  the  Galaxy 
arises  from  the  light  of  countless  stars,  too  minute  to  be  sep- 
arately visible  with  the  naked  eye.  We  find,  then,  that  the 
telescopic  stars,  instead  of  being  divided  up  into  a  limited 
number  of  constellations,  are  mostly  condensed  in  the  region 


DESCRIPTION  OF  THE  PltiyCIPAL  COXSTELLATIOXS.      417 

of  tlie  Galaxy.  They  are  least  numerous  in  the  regions  most 
distant  from  the  galactic  belt,  and  grow  thicker  as  we  aj)- 
proach  it.  The  more  powerful  the  telescope,  the  more  marked 
the  condensation  is.  AVith  the  naked  e^'e,  the  condensation  is 
hardly  noticeable,  unless  by  actual  count :  a  very  small  tele- 
scope will  show  a  decided  thickening  of  the  stars  in  and  near 
the  Galaxy;  while,  if  we  employ  the  most  powerful  telescoj)e8, 
a  large  majoi-ity  of  the  stars  they  show  are  found  to  lie  act- 
ually in  the  Galaxy.  In  other  words,  if  we  should  blot  out 
all  the  stars  visible  with  a  twelve-inch  telescope,  we  should 
find  that  the  greater  part  of  the  remaining  stars  were  in  the 
Galaxy.  The  structme  of  the  universe  which  this  fact  seems 
to  indicate  will  be  explained  in  a  subsequent  section. 

Clusters.  —  Besides  tliis  gradual  and  regular  condensation 
towards  the  galactic  belt,  occasional  condensations  of  stars 
into  clusters  may  be  seen.  Indeed,  some  of  these  clusters  are 
visible  to  the  naked  eye,  sometimes  as  separate  stars,  like  the 
Pleiades,  but  more  commonly  as  milky  patches  of  lipiit,  be- 
cause the  stars  are  too  small  to  be  seen  separately.  The  num- 
ber visible  in  powerful  telescopes  is,  however,  much  greater. 
Sometimes  there  are  hundreds,  or  even  thousands,  of  stars  visi- 
ble in  the  field  of  the  telescope  at  once;  and  sometimes  the 
number  is  so  great,  and  the  individual  stars  so  small,  that  they 
cannot  be  counted  even  in  the  most  powerful  telescopes  ever 
made. 

Nebula. — Another  class  of  objects  which  are  found  in  the 
celestial  spaces  are  irregular  masses  of  soft,  cloudy  light, 
which  are  hence  termed  nebulae.  Many  objects  which  look 
like  nebula3  in  small  telescopes  are  found  by  mora  powerful 
ones  to  be  really  star  clusters.  But,  as  we  ^hall  hereafter 
show,  many  of  these  objects  are  not  composed  of  stars  at  all, 
but  of  immense  masses  of  gaseous  matter, 

§  2.  Descyij)tioii  of  the  Principal  Constellations. 

For  the  benefit  of  the  reader  who  wishes  to  make  himself 
acquainted  with  the  constellations  in  detail,  or  to  identify  any 
bright  star  or  constellation  which  he  may  see,  we  present  a 

28 


418  THE  STELLAR   UNIVERSE. 

brief  description  of  the  principal  objects  wliich  may  be  seen 
in  the  lioavens  at  different  seasons,  ilhistrated  by  five  maps, 


showinij  the  stars  to  the  Hftli  majirnitude  inclusive.  Tlie 
reader  who  does  not  wisli  to  enter  into  tliese  details  can  pass 
to  tlie  next  section  without  any  break  of  the  continuity  of 
thouglit. 

For  the  purpose  of  learning  the  constellations,  the  star- 
maps  will  be  a  valuable  auxiliary.  It  will  be  better  to  begin 
with  tlie  northern,  or  circunipolar,  constellations,  because  these 
are  nearly  always  visible  in  our  latitude.  The  iirst  one  to  be 
looked  for  is  Ursa  Major  (the  Great  Bear,  or  the  Dipper),  from 
which  the  pole  star  can  always  be  found  by  means  of  the 
pointers,  as  shown  in  Fig.  2,  page  10.  Supposing  the  observer 
to  look  for  it  at  nine  o'clock  in  the  evening,  he  will  see  it  in 
various  positions,  depending  on  the  time  of  year,  namely,  in 

April  and  May north  of  the  zenith. 

July  and  August to  the  west  of  north,  the  pointers  lowest. 

October  and  November close  to  the  north  horizon. 

January  and  February to  the  east  of  north,  the  pointers  highest. 

These  successive  positions  are  in  the  same  order  with  those 
which  the  constellation  occupies  in  consequence  of  its  diurnal 
motion  around  the  pole.  The  pointers  are  in  the  body  of  the 
bear,  while  the  row  of  stars  on  the  other  end  of  the  constella- 
tion forms  his  tail. 

Ursa  Minor,  or  the  Little  Dipper,  is  the  constellation  to 
which  the  pole  star  belongs.  It  includes,  besides  the  pole 
star,  another  star  of  the  second  magnitude,  which  lies  nearly 
in  the  direction  of  the  tail  of  Ursa  Major. 

Cassiopeia,  or  the  Lady  in  the  Chair,  is  on  the  opposite  side 
of  the  pole  from  Ursa  Major,  at  nearly  the  same  distance. 
The  constellation  can  be  readily  recognized  from  its  three  or 
four  bright  stars,  disposed  in  a  line  broken  into  pieces  at  right 
angles  to  each  other.  In  the  ancient  mythology,  Cassiopeia  is 
the  queen  of  Cepheus ;  and  in  the  constellation  she  is  repre- 
sented as  seated  in  a  large  chair  or  throne,  from  which  she  is 
issuing  her  edicts. 

Perseus  is  quite  a  brilliant  constellation,  situated  in  the 


DESCRIVTION  OF  THE  I'lilXCIPAL  CONSTELLATIONS.     419 

Milky  Way,  east*  of  Cassiopeia,  and  a  little  farther  from  the 
pole.  It  may  be  recognized  by  a  row  of  conspicuous  stars 
extending  along  the  Milky  Way,  which  passes  directly  through 
this  constellation. 

Other  circumpolar  constellations  are  Cepheus,  the  Camelo- 
pard,  tlie  Lynx,  the  Di-agon  {Draco),  and  the  Lizard ;  but  they 
do  not  contain  any  stars  so  bright  as  to  attract  especial  atten- 
tion. The  reader  who  wishes  to  learn  them  can  easily  lind 
them  by  comparing  the  star-maps  with  the  heavens. 

Owing  to  the  annual  motion  of  the  sun  among  the  stars,  the 
constellations  which  are  moi-e  distant  from  the  pole  cannot  be 
seen  at  all  times,  but  must  be  looked  for  at  certain  seasons, 
unless  inconvenient  hours  of  the  night  be  chosen.  We  shall 
describe  the  more  remarkable  constellations  as  they  are  seen 
by  an  observer  in  middle  north  latitudes  in  four  different 
positions  of  the  starry  sphere.  The  sphere  takes  all  four  of 
these  positions  every  day,  by  its  diurnal  motion;  but  some  of 
these  positions  will  occur  in  the  daytime,  and  others  late  at 
night  or  early  ii  the  morning. 

First  Position^  Orion  on  the  Meridian.  —  The  constellations 
south  of  the  zenith  are  those  shown  on  Maps  IL  and  IIL,  the 
former  being  west  of  the  meridian,  the  latter  east  This  posi- 
tion occurs  on 

December  2Ist at  midnight. 

Jaiiuiuy  21st at  10  o'clock  p.m. 

February  20tli at  8  o'clock  p.m. 

March  21st at  6  o'clock  p.m. 

And  80  on  through  the  year.  In  this  position,  Cassiopeia  and 
Ursa  Major  are  near  the  satne  altitude,  the  former  higii  up  in 

*  In  the  celestial  sphere  the  points  of  the  compass  have,  of  necessity,  a  mean- 
ing wliich  nuiy  seem  ditierent  from  that  which  we  attribute  to  tiiem  on  the  earth. 
North  always  means  towards  the  north  pole ;  south,  from  it ;  icest,  in  the  direc- 
tion of  the  diurnal  motion  ;  east,  in  the  opposite  direction.  In  Fig.  2,  tlie  arrows 
all  jioint  west,  and  by  examining  the  figure  it  will  be  seen  tiuit  below  the  pole 
north  is  upwards,  and  cast  is  towards  the  west  horizon.  Really,  these  definitions 
hold  equally  true  for  the  earth,  the  same  differences  being  found  between  the 
points  of  the  compass  at  different  places  on  the  earth — here  and  in  China,  for  in- 
stance— that  we  see  on  the  celestial  sphere. 


420  THE  STELLAR   UNIVERSE. 

the  north-west,  the  latter  in  the  north-east.  The  Milky  Way 
spans  the  heavens  like  an  arch,  resting  on  the  horizon  in  the 
north-north-west  and  south-soutli-east.  We  shall  lirst  describe 
the  constellations  in  its  course. 

Cygmis,  the  Swan,  is  sinking  below  the  horizon,  where  the 
Milky  Way  rests  npon  it  in  the  north-north-west,  and  only  a 
few  stars  of  it  are  visible.  It  will  be  better  seen  at  another 
season. 

Kext  in  order  come  Cephens,  Cassiopeia,  and  Perseus,  which 
we  have  already  described  as  circunipolar  constellations. 

Above  Perseus  lies  Auruja,  the  Charioteer,  which  may  be 
readily  recognized  by  a  bright  star  of  the  first  magnitude, 
called  Capella,  the  Goat,  now  a  few  degrees  north-west  of  the 
zenith.  Auriga  is  represented  as  holding  a  goat  in  his  arm, 
in  the  body  of  which  this  star  is  situated.  About  ten  degrees 
east  of  Capella  is  the  star  /3  Aurigai  of  the  second  magnitude ; 
while  still  farther  to  the  east  is  a  group  of  small  stars  which 
also  belongs  to  the  same  constellation.  The  latter  extends 
some  distance  south  of  the  zenith. 

The  Milky  Way  next  passes  between  Taurus  and  Gemini, 
which  we  will  describe  presently,  and  then  crosses  the  equator 
east  of  Orion,  the  most  brilliant  constellation  in  the  heavens, 
having  two  stars  of  the  iirst  magnitude  and  four  of  the  second. 
The  former  are  Betelguese,  or  a  Orionis,  which  is  highest  up, 
and  may  be  recognized  by  its  reddish  color,  and  Kigel,  or  j3 
Orionis,  a  sparkling  white  star,  lower  down,  and  a  little  to  the 
west.  The  former  is  in  the  shoulder  of  the  figure,  the  latter 
in  the  foot.  Between  the  two,  three  stars  of  the  second  mag- 
nitude, in  a  row,  form  the  l)elt  of  the  warrior. 

Canis  Miiio)',  the  Little  Dog,  lies  just  across  the  Milky  Way 
from  Orion,  and  may  be  recognized  by  the  bright  star  Pro- 
cyon,  of  the  first  magnitudti,  due  east  from  Petelguese. 

Canis  Majoi\  tiie  Great  Dog,  lies  south-east  of  Orion,  and  is 
easily  recognized  by  Sirius,  the  brightest  fixed  star  in  the  heav- 
ens. A  number  of  bright  stars  south  and  south-east  of  Sirius 
belong  to  this  constellation,  making  it  one  of  great  brilliancy. 

As  the  Milky  Way  approaches  the  south  horizon,  it  passes 


DESCRIPTION  OF  THE  PRINCirAL  CONSTELLATIONS.     421 

through  Argo  Ncivis,  the  Sliip  Argo,  which  is  partly  below  the 
horizon.  It  contains  Canopns,  the  next  brightest  star  to  Siri- 
ng ;  but  this  object  is  below  the  horizon,  unless  the  observer  is 
as  far  south  as  35°  of  north  latitude. 

We  can  next  trace  such  of  the  zodiacal  constellations  as  are 
high  enough  above  the  horizon.  In  the  west,  one-tliird  of  the 
way  fi'oin  the  horizon  to  the  zenith,  will  be  seen  Aries,  the 
Ram,  which  may  be  recognized  by  three  stars  of  the  second, 
tliird,  and  fourth  magnitudes,  respectively,  forming  an  obtuse- 
angled  triangle,  the  brightest  star  being  the  highest.  The 
arrangement  of  these  stars,  and  of  some  others  of  the  fifth 
magnitude,  may  be  seen  by  Map  II. 

Ihurus,  the  Bull,  is  next  above  Aries,  and  may  be  recog- 
nized by  the  Pleiades,  or  "  seven  stars,"  as  the  group  is  com- 
monly called.  Ileally  there  are  only  six  stars  in  the  grouj) 
clearly  visible  to  ordinary  eyes,  and  an  eye  which  is  good 
enough  to  see  seven  will  be  likely  to  see  four  others,  or  eleven 
in  all.  A  telescopic  view  of  this  group  will  be  given  in  con- 
nection with  the  subject  of  clusters  of  stars.  Another  group 
in  this  constellation  is  the  Ilyades,  the  principal  stars  of  which 
are  arranf^cd  in  the  form  of  the  letter  V,  one  exti'emitv  of  the 
V  being  formed  b}'  Aldebaran,  a  red  star  ranked  as  of  the 
first  magnitude,  but  not  so  bright  as  a  Orionis, 

Gcniini,  the  Twins,  lies  east  of  the  Milky  Way,  and  may  be 
found  on  the  left  side  of  Map  II.  and  the  right  of  Map  III. 
The  brightest  stars  of  this  constellation  are  Castor  and  Pollux, 
or  a  and  (5,  which  lie  twenty  or  thirty  degrees  south-east  or 
east  of  the  zenith,  about  one-fourth  or  one-third  of  the  way 
to  the  horizon.  They  are  almost  due  north  from  Procyon ; 
that  is,  a  line  drawn  from  Procyon  to  the  pole  star  passes  be- 
tween them.  The  constellation  extends  from  Castor  and  Pol- 
lux some  distance  south  and  Avest  to  the  borders  of  Orion. 

Cance}\  the  Crab,  lies  east  of  Gemini,  but  contains  no  bright 
star.  The  most  noteworthy  object  within  its  borders  is  Prai- 
sejie,  a  group  of  stars  too  small  to  be  seen  singly,  which  ap- 
pears as  a  R]>ot  of  milky  light.  To  see  it  well,  the  night  must 
be  perfectly  clear,  and  the  moon  not  in  the  neighborhood. 


422  THE  STELLAR   UNIVERSE. 

Leo,  tlie  Lion,  contains  the  bright  star  Regnhis,  about  two 
hours  above  the  eastern  horizon.  This  star,  witli  five  or  six 
smaller  ones,  forms  a  sickle.  Regains  being  the  handle.  The 
sickle  is  represented  as  in  the  breast,  neck,  and  head  of  the 
lion,  his  tail  extending  nearly  to  the  horizon,  where  it  ends  at 
the  star  Denebola,  now  just  risen. 

Such  are  the  principal  constellations  visible  in  the  supposed 
position  of  the  celestial  sphere.  If  the  hour  of  observation  is 
different  from  that  supposed,  the  positions  of  the  constellations 
will  be  different  by  the  amount  of  diurnal  rotation  during  the 
interval.  For  instance,  if,  in  the  middle  of  March,  we  study 
the  heavens  at  eight  o'clock  instead  of  six,  the  western  stars 
will  be  nearer  the  horizon,  the  southern  ones  farther  west,  and 
the  eastern  ones  higher  up  than  w^e  have  described  them. 

Second  Position  of  the  Celestial  Sphere.  —  The  meridian  in 
twelve  hours  of  nVht  ascension,  near  the  left-hand  edire  of 
Map  III.,  and  the  right-hand  edge  of  Map  lY.  The  stars  on 
Map  III.  are  west  of  the  meridian,  those  of  Map  IV.  east  of  it. 
This  position  occurs  on 

March  21st at  midnight. 

April  20th at  10  o'clock. 

May  21st at  8  o'clock. 

In  this  position  Ursa  Major  is  near  the  zenith,  and  Cassiopeia 
in  the  north  horizon.  The  Milky  Way  is  too  near  the  horizon 
to  be  visible ;  Orion  has  set  in  the  west ;  and  there  are  no  very 
conspicuous  constellations  in  the  south.  Castor  and  Pollux  are 
visible  in  the  north-west,  at  a  considerable  altitude,  and  Pro- 
cyon  in  the  west,  about  an  hour  and  a  half  above  the  horizon. 
Leo  is  west  of  the  meridian,  extending  nearly  to  it,  while  three 
new  zodiacal  constellations  liave  come  into  sight  in  the  east. 

Virgo,  the  Virgin,  has  a  single  bright  star — Spica — about 
the  brilliancy  of  Regulus,  now  about  one  hour  east  of  the  me- 
ridian, and  a  little  more  than  half-way  from  the  zenith  to  the 
horizon. 

Libra,  tlie  Balance,  has  no  stars  which  will  attract  attention. 
The  constellation  may  be  recognized  by  its  position  between 
Virgo  and  Scorpius. 


DESCRIPTION  OF  THE  PRINCirAL  CONSTELLATIONS.     423 

^orpins,  the  Scorpion,  is  just  rising  in  the  south-east,  and  is 
not  yet  high  enough  to  be  well  seen. 

Among  the  constellations  north  of  the  zodiac  we  have: 

Coma  Berenices,  the  Hair  of  Berenice,  now  exactly  on  the 
meridian,  and  about  ten  degrees  south  of  the  zenith.  It  is  a 
close,  irregular  group  of  very  small  stars,  quite  different  from 
anything  else  in  the  heavens.  In  the  ancient  mythology,  Ber- 
enice had  vowed  her  hair  to  the  goddess  Venus ;  but  Jupiter 
carried  it  away  from  the  tehij)le  in  which  it  was  deposited, 
and  made  it  into  a  constellation. 

Bootes,  the  Bear-keeper,  is  a  large  constellation  east  of  Coma. 
It  is  marked  by  A  returns,  a  very  bright  but  somewhat  red 
star,  an  hour  and  a  half  cast  of  Coma  Berenices. 

Ccmes  Venatici,  the  Hunting  Dogs,  are  north  of  Coma.  They 
are  held  in  a  leash  by  Bootes,  and  are  chasing  Ursa  Major 
round  the  pole. 

Corona  Borealis,  the  Northern  Crown,  lies  next  east  of  Bootes 
in  the  north-east.  It  is  principally  composed  of  a  pretty  semi- 
circle of  stars,  supposed  to  form  a  chaplet,  or  crown. 

Third  Position  of  the  Sphere. — The  southern  constellations 
are  those  shown  on  Mapt  IV.  and  V.,  those  of  Map  IV.  being 
west  of  the  meridian,  and  those  of  Map  V.  east  of  it.  This 
position  occurs  on 

June  21  st <it  midnight. 

July  'Jlst at  10  o'clock. 

August  21st at  8  o'clock. 

etc etc. 

In  this  position  the  Milky  Way  is  once  more  in  sight,  and 
seems  to  span  the  heavens,  but  we  do  not  see  the  same  ])ai't 
of  it  which  was  visible  in  the  first  position.  Cassiopeia  is 
now  in  the  north-east,  and  Ursa  Major  has  passed  over  to  the 
north-west.  Arcturus  is  two  or  three  hours  liigh  in  the  west, 
and  Corona  is  above  it,  two  or  three  hours  west  of  the  zenith. 
Commencing,  as  in  the  first  position,  with  the  constellations 
which  lie  along  the  Milky  Way,  we  start  upwards  from  Cas- 
siopeia, pass  Cepheus  and  Laccrta,  neither  of  which  contains 
any  striking  stars,  and  then  reach 


424  THE  STELLAR   UNIVERSE. 

Ci/gnus,  the  Swan,  now  north-east  from  the  zenith,  which 
may  be  recognized  by  four  or  five  stars  forming  a  cross,  di- 
rectly in  the  Milky  Way.  The  brightest  of  these  stars  some- 
what exceeds  the  brightest  ones  of  Cassiopeia. 

Lt/m,  the  Harp,  is  west  and  soutli-west  of  Cygniis,  and  near 
the  zenith.  It  contains  the  bright  star  Vega,  or  a  Lyrse,  of 
the  first  magnitude,  of  a  brilliant  white  color  with  a  tinge  of 
blue. 

Passing  south,  over  Vidpecida,  the  Little  Fox,  and  Sagilta, 
the  Arrow,  the  next  striking  constellation  we  reach  is 

Aqiiila,  the  Eagle,  now  midway  between  the  zenith  and  the 
horizon,  and  two  hours  east  of  the  meridian.  It  contains  a 
bright  star  —  Altair,  or  a  Aquilai  —  situated  between  two 
smaller  ones,  the  row  of  three  stars  running  nearly  north  and 
south. 

We  next  pass  west  of  the  Milky  Way,  and  direct  our  atten- 
tion to  a  point  two  hours  west  of  the  meridian,  and  some  dis- 
tance towards  the  south  horizon.     Here  we  find 

iScorjnus,  the  Scorpion,  a  zodiacal  constellation  and  a  quite 
brilliant  one,  containing  A)dares,  or  a  Scorpii,  a  reddish  star 
of  nearly  the  first  magnitude,  with  a  smaller  star  on  each  side 
of  it,  and  a  long  curved  row  of  stars  to  the  west. 

tS(((/it{arius,  the  Archer,  comprises  a  large  collection  of  sec- 
ond-magnitude stars  east  of  Scorpius,  and  in  and  east  of  the 
Milky  Way,  and  now  extending  from  the  meridian  to  a  point 
two  iiours  east  of  it. 

Capricornus,  the  Goat,  another  zodiacal  constellation,  is  now 
in  the  south-east,  but  contains  no  striking  stars.  The  same 
remark  applies  to  Aquarius,  the  Water-bearer,  which  has  just 
risen,  and  Pisces,  the  Fishes,  partly  below  the  eastern  horizon. 

Leaving  the  zodiac  again,  we  find,  north  of  Scorpius  and 
west  of  the  Milky  Way,  a  very  large  pair  of  constellations, 
called  OpJi'mclnis,  the  Serpent-bearer,  and  Serpens,  the  Serpent. 
Ophinchus  stands  with  one  foot  on  Scorpius,  while  his  head  is 
marked  by  a  star  of  the  second  magnitude  twelve  degrees 
north  <)f  the  equator,  and  now  on  the  meridian.  It  is,  there- 
fore, one-third  or  one-fourth  of  the  way  from  the  zenith  to  the 


DESCRirnON  OF  THE  PRINCIPAL  CONSTELLATIONS.     425 

horizon.  The  Serpent,  which  he  holds  in  his  hands,  lies  with 
its  tail  in  an  opening  of  the  Milky  Way,  south-west  of  Aquila, 
while  its  neck  and  head  are  formed  by  a  collection  of  stars  of 
the  second,  third,  and  fourth  magnitudes  some  distance  north 
of  Scorpius,  and  extending  up  to  the  borders  of  Bootes. 

Hercules  is  a  very  large  constellation,  bounded  by  Corona 
on  the  west,  Lyra  on  the  east,  Ophiuchus  on  the  south,  and 
Draco  on  the  north.  It  is  now  in  the  zenith,  but  contains  no 
striking  stars. 

i>mco,  the  Dragon,  lies  with  his  head  just  north  of  Hercules, 
while  his  body  is  marked  b}'  a  long  curved  row  of  stars  ex- 
tending round  the  pole  between  the  Great  and  the  Little  Bear, 
llis  head  is  readily  recognized  by  a  collection  of  stars  of  the 
second  and  third  magnitudes  which  might  well  suggest  such 
an  objec' 

Fourtlt  Position  of  the  Sphere. — The  southern  constellations 
are  now  found  on  Maps  V.  and  II. — those  of  Map  V.  west  of 
the  meridian,  those  of  Map  II.  east  of  it.     The  times  are : 

September  21st at  midnight. 

October  21st = at  10  o'clock. 

November  2()th at  8  o'clock. 

December  21st at  G  o'clock. 

In  this  position  Cassiopeia  is  just  north  of  the  zenith,  while 
Ursa  Major  is  glimmering  in  the  north  horizon.  Following 
the  Milky  Way  from  Cassiopeia  towards  the  west,  we  shall 
cross  Cepheus,  Cygnus,  Lyra,  and  Aipiila,  while  towards  the 
east  we  pass  Perseus  and  Auriga,  all  of  which  have  been  de- 
scribed. 

In  the  south,  the  principal  constellation  is  Pegasus,  the  Fly- 
ing Horse,  distinguished  by  four  stars  of  the  second  magni- 
tude, which  form  a  large  square,  each  side  of  which  is  about 
fourteen  degrees. 

Andromeda,  her  hands  in  chains,  is  readily  found  by  a  row 
of  three  bright  stars  extendins:  north-east  from  the  north-east 
corner  of  Pegasus  in  the  direction  of  Perseus. 

Celus,  the  Whale,  is  a  large  constellation  in  the  south,  ex- 
tending from  the  meridian  to  a  point  three  hours  east  of  it. 


426  THE  STELLAR   UNIVEESE. 

Its  briglitest  stars  are  ft  Ceti,  now  near  tlie  meridian,  at  an  al- 
titude of  20°,  which  stands  by  itself,  and  a  Ceti,  about  20°  be- 
low Aries,  wliich  is  now  about  30°  south-east  from  the  zenitli. 
The   reader  who   wishes  to   consult  the   constellations  in 
greater  detail  can  readily  do  so  by  means  of  tlie  star-maps. 

§  3.  Neiu  and  Variable  Stars. 

Tlie  large  majority  of  stars  always  apjiear  to  be  of  the  same 
brightness,  though  it  is  quite  possible  that,  if  the  quantity  of 
light  emitted  by  a  star  could  be  measured  with  entire  preci- 
sion, it  would  be  found  in  all  cases  to  vary  slightly,  from  time 
to  time.  There  are,  however,  quite  a  number  of  stars  in  which 
the  variation  is  so  decided  that  it  has  been  detected  by  com- 
paring  their  apparent  brightness  with  that  of  other  stars  at  dif- 
ferent times.  More  than  a  hundred  such  stars  are  now  known ; 
but  in  a  large  majority  of  cases  the  variation  is  so  slight  that 
only  careful  observation  with  a  practised  eye  can  perceive  it. 
There  are,  however,  two  stars  in  which  it  is  so  decided  that 
the  most  casual  observer  has  onl}'  to  look  at  the  proper  times, 
in  order  to  see  it.  These  are  ft  Persei  and  o  Ceti,  or  Algol 
and  Mira,  to  which  we  might  add  rj  Argus,  a  star  of  the  south- 
ern hemisphere,  which  exhibits  variations  of  a  very  striking 
character. 

Variations  of  Algol. —  This  star,  marked  ft  in  the  constel- 
lation Perseus,  may  be  readily  found  on  Maps  I.  and  II.,  in 
right  ascension  3  hours  and  declination  40°  23'.  When  once 
found,  it  is  readily  recognized  by  its  position  nearly  in  a  line 
between  two  smaller  stars.  The  most  favorable  seasons  for 
seeing  it  in  the  early  evening  are  the  autumn,  winter,  and 
spring.  In  autumn  it  will,  after  sunset,  generally  be  low 
down  in  the  north-cast ;  in  winter,  high  up  in  the  north,  not 
far  from  the  zenith;  and  in  spring,  low  down  in  the  north- 
west. Usually  it  shines  as  a  faint  second-magnitude  star:  on 
an  accurate  scale  the  magnitude  is  al)out  2|^.  Put  at  inter- 
vals of  a  little  less  than  three  da^'s,  it  fades  out  to  the  fourth 
magnitude  for  a  few  hours,  and  then  resumes  its  usual  splen- 
dor once  more.     These  changes  were  first  noticed  about  two 


NEW  AND   VARIABLE  STARS.  427 

centuries  ago,  but  it  was  not  till  1782  that  tliey  were  accu- 
rately observed.  The  period  is  now  known  to  be  2  days,  20 
hours,  49  minutes — that  is,  3  hours  11  minutes  less  than  three 
days.  It  takes  about  four  hours  and  a  half  to  fade  away  to 
its  least  brilliancy,  and  four  hours  more  are  spent  in  recover- 
ing its  light;  so  that  there  are  nine  and  a  half  hours  during 
each  period  in  which  its  light  is  below  the  average.  But  near 
the  beginning  and  end  of  the  variations,  the  change  is  very 
slow,  so  that  there  are  not  more  thaji  five  or  six  hours  during 
which  the  ordinary  eye  would  see  that  the  star  was  any  smaller 
than  usual. 

The  apparent  regularity  of  this  variation  of  light  at  iirst 
suggested,  as  an  explaiuition  of  its  cause,  that  a  large  dark 
planet  was  revolving  round  Algol,  and  passed  over  its  face 
at  every  revolution,  thus  cutting  off  a  portion  of  its  light. 
This  theory  accounts  very  well  for  the  salient  features  of 
the  variation.  But  when  the  latter  came  to  be  studied  more 
closely  and  carefully,  it  was  found  that  there  were  small  irreg- 
ularities in  the  variation  which  the  theory  would  not  well  ac- 
count for.  The  period  of  the  variation  was  found  to  change  a 
little  at  different  times,  while  the  star  does  not  lose  and  recover 
its  light  in  the  same  time  as  it  would  if  the  passage  of  a  dark 
body  caused  the  changes. 

Another  remarkable  variable  star,  but  of  an  entirely  differ- 
ent type,  is  o  Ceti,  or  Mira  (the  Wonderful).  It  may  be  found 
on  Map  II.,  in  right  ascension  2  hours  12  minutes,  declination 
3°  39'  south.  During  most  of  the  time  this  star  is  entirely 
invisible  to  the  naked  eye,  but  at  intervals  of  about  eleven 
months  it  shines  forth  with  the  brilliancy  of  a  star  of  the  sec- 
ond or  third  magnitude.  It  is,  on  the  average,  about  forty 
davs  fro'.n  the  time  it  first  becomes  visible  until  it  attains  its 
greatest  brightness,  and  it  then  requires  about  two  months  to 
become  invisible ;  so  that  it  comes  into  sight  more  rapidly 
than  it  fades  away.  It  is  expected  to  attain  its  greatest  brill- 
iancy in  November,  1877 ;  in  October,  1878,  and  so  on,  about 
a  month  earlier  each  year;  but  the  period  is  quite  irregular, 
ranging  from  ten  to  twelve  months,  so  that  the  times  of  its 


428  T^^  STELLAR   UNIVERSE. 

aj)pearance  cannot  be  predicted  with  certainty.  Its  maximum 
brilliancy  is  also  variable,  being  sometimes  of  the  second  mag- 
nitude, and  at  others  only  of  the  third  or  fourth. 

r\  Aryiis. — Perhaps  the  uiost  extraordinary  known  variable 
star  in  the  heavens  is  »/  Argus,  of  the  southern  hemisphere,  of 
which  the  position  is,  right  ascension,  10  hours  40  minutes ; 
declination,  59°  1'  south  Being  so  far  south  of  the  equator, 
it  cannot  be  seen  in  our  latitudes,  and  the  discovery  and  ob- 
ser\ations  of  the  variations  of  its  light  have  been  generally 
made  by  astronomers  who  have  visited  the  southern  hemi- 
sphere. In  1677,  Ilalley,  while  at  St.  Helena,  found  it  to  be 
of  the  fourth  magnitude.  In  1751,  Lacaille  found  that  it  had 
increased  to  the  second  magnitude.  From  1828  to  1838  it 
ranged  between  the  first  and  second  magnitudes.  The  iirst 
careful  observations  of  its  variability  were  made  by  Sir  John 
Herschel  while  at  the  Cape  of  Good  Hope.  He  says :  "  It 
was  on  the  IGth  December,  1837,  that,  resuming  the  photo- 
metrical  comparisons,  my  astonishment  was  excited  by  the  ap- 
pearance of  a  new  candidate  for  distinction  among  the  very 
brightest  stars  of  the  first  magnitude  in  a  part  of  the  heav- 
ens with  which,  being  perfectly  familiar,  I  was  certain  that  no 
such  brilliant  object  had  before  been  seen.  After  a  momen- 
tary hesitation,  the  natural  consequence  of  a  phenomenon  so 
utterly  unexpected,  and  referring  to  a  map  for  its  configura- 
tion with  other  conspicuous  stars  in  the  neighborhood,  I  be- 
came satisfied  of  its  identity  with  my  old  acquaintance,  t/  Ar- 
gus. Its  light,  was,  however,  nearly  tripled.  While  yet  low, 
it  equalled  Rigel,  and,  when  it  attained  some  altitude,  was 
decidedly  greater."*  Sir  John  states  that  it  continued  to  in- 
crease until  January  2d,  1838,  when  it  was  nearly  matched 
with  a  Centauri.  It  then  faded  a  little  till  the  close  of  his 
observations  in  April  following,  but  was  still  as  bright  as  Al- 
debaran.  But  in  1842  and  1843  it  blazed  up  brighter  than 
ever,  and  in  March  of  the  latter  year  was  second  only  to 
Sirius.     During  the  twenty-five  years  following,  it  slowly  but 

•  "Astronomical  Observations  at  the  Cape  of  Good  Hope,"  p.  33. 


NEW  AND   VAEIAIiLE  STAIIS.  429 

steadily  diminished :  in  1867  it  was  barely  visible  to  the  naked 
eye,  and  tlie  year  following  it  vanished  entirely  from  the  un- 
assisted view,  and  has  not  yet  begun  to  recover  its  brightness. 

When  we  speak  of  this  star  as  the  most  remarkable  of  the 
well-known  variables,  we  refer,  not  to  the  mere  range  of  its 
variations,  but  to  its  brilliancy  when  at  its  maximum.  Sev- 
eral cases  of  equally  great  variation  are  known  ;  but  the  stars 
are  not  so  bright,  and  therefore  would  not  excite  so  much  no- 
tice. Thus,  the  star  11  Andromeda)  varies  from  the  sixth  to 
the  thirteenth  magnitude  in  a  pretty  regular  period  of  405 
days.  When  at  its  brightest,  it  is  just  visible  to  the  naked 
eye,  while  only  a  large  telescope  will  show  it  when  at  its  min- 
imum. A  number  of  others  ran<Te  througli  five  or  six  orders 
of  magnitude,  but  o  Ceti  is  the  only  one  of  these  which  ever 
l>ecomes  as  bright  as  the  second  magnitude. 

The  foregoing  stars  are  the  only  ones  the  variations  of 
which  would  strike  the  ordinary  observer.  Among  the  liun- 
dred  remaining  ones  which  astronomers  have  noticed,  /3  Lyra) 
is  remarkable  for  having  two  maxima  and  two  minima  of  un- 
ecpial  brilliancy.  If  we  take  it  when  at  its  greatest  minimum, 
we  lind  its  magnitude  to  be  4^.  In  the  course  of  three  days, 
it  will  rise  to  magnitude  3^.  In  the  course  of  the  week  fol- 
lowing, it  will  first  fall  to  the  fourth  magnitude,  and  increase 
again  to  magnitude  S^.  In  three  days  more  it  will  dro}) 
again  to  its  minimum  of  magnitude  4^;  the  period  in  which 
it  goes  through  all  its  changes  being  thirteen  days.  This  pe- 
riod is  constantly  increasing.  The  changes  of  this  star  can 
best  be  seen  by  comparing  it  with  its  neighbor,  y  Lyra).  Some- 
times it  will  appear  equally  bright  with  the  latter,  and  at  other 
times  a  magnitude  smaller.* 

*  In  ISTf),  Professor  Schonfeld,  now  director  of  the  observatory  at  Bonn,  pub- 
lisbed  a  complete  catalogue  of  known  variable  stars,  the  total  number  being  143. 
The  following  are  the  more  remarkable  ones  of  his  list.  The  positions  are  re- 
ferred to  the  ecliptic  and  equinox  of  187") : 

T  Cassiopeia! :  right  ascension,  0  hours  1(5  minutes  2J)  seconds;  declination,  5r)° 
f'/.O  N. — This  is  a  case  in  which  a  star,  having  once  been  observed,  was  after- 
wards found  to  be  missing.  Examination  showed  that  it  had  so  far  diminished 
as  to  be  no  longer  visible  without  a  larger  telescope,  and  continued  observations 


430  THE  STELLAR   UNIVERSE. 

New  Stars. — It  was  onco  supposed  to  bo  no  uncommon  occur- 
rence for  new  stars  to  come  into  existence  and  old  ones  to  dis- 
appear, tlie  former  being  looked  upon  as  new  creations,  and 
the  di8ap})earances  as  due  to  the  destruction  or  anniliilation 
of  those  stars  which  had  fulfilled  their  end  in  the  economy  of 
nature.  The  8Upj)osed  disappearances  of  stars  are,  however, 
found  to  have  no  certain  foundation  in  fact,  probably  owing 
their  origin  to  errors  in  recording  the  position  of  stars  actu- 
ully  existing.  It  was  explained,  in  treating  of  Practical  As- 
troaomy,  that  the  astronomer  determines  the  position  of  a 
body  in  the  celestial  vault  by  observing  the  clock-time  at  which 
it  passes  the  meridian,  and  the  position  of  the  circle  of  bis  in- 


showed  it  to  range  from  tlie  seventh  to  the  eleventh  magnitude  with  a  regular 
period  of  4I5G  days. 

B  Cassiopeia; :  right  ascension,  0  hours  17  minutes  52  seconds ;  declination, 
();3°  27. 0  N. — Tiiis  is  supposed  to  he  tlie  celebrated  star  which  blazed  out  in 
November,  ir>72,  and  was  so  fully  described  by  Tycho  Brahe.  But  tiie  proof  of 
identity  can  hardly  be  considered  conclusive,  especially  as  no  variation  has,  of  re- 
cent years,  been  noticed  in  the  star. 

o  Ceti :  right  ascension,  2  hours  i;{  minutes  1  second;  declination,  3°  32'. 7 
S. — We  have  alri^ady  described  the  variations  of  this  star. 

/8  Persei,  or  Algol  :  right  ascension,  8  hours  0  minutes  2  seCDnds  ;  declina- 
tion, 40^  28'.4  N. — The  variations  of  this  star,  which  is  the  most  regular  one 
known,  have  just  been  described. 

11  Atnigie :  right  ascension,  a  hours  7  minutes  12  seconds;  declination,  53° 
26'. fi  N, — This  star  is  one  of  very  wide  and  complex  variation,  changing  from  the 
sixth  to  the  thirteenth  maguitude  in  a  period  of  about  40")  days. 

11  Geminorum :  right  ascension,  (J  hours  51)  minutes  4!)  seconds ;  declination, 
22°  53'.8  N. — This  star  was  discovered  by  Mr.  Hind,  of  Eugland,  and  ranges  be- 
tween the  seventh  and  the  twelfth  magnitude  in  a  period  of  371  days. 

U  Geminorum :  right  ascension,  7  hours  47  minutes  41  seconds ;  declination, 
22°  1!)'.7  N. — An  irregidar  variable,  never  visible  to  the  naked  eye,  remarkable 
for  the  rapidity  with  which  it  sometimes  changes.  SchiJnfeld  says  that  in  Feb- 
ruary, 18(;0,  it  increased  three  entire  magnitudes  in  24  hours.  The  periods  of  its 
greatest  brightness  have  ranged  from  75  to  (!  1 7  days. 

t]  Argus:  right  ascension,  10  hours  40  minutes  13  seconds;  declination,  59° 
I'.G  S. — This  remarkable  object  has  already  been  described. 

II  llydnc :  right  ascension,  13  hours  22  minutes  53  seconds;  declination,  22° 
38'. 0  S. — The  variability  of  this  star  was  recognized  by  Maraldi,  in  1704.  It  is 
generally  invisible  to  the  naked  eye,  but  rises  to  about  the  fiftii  magnitude  at 
intervals  of  about  437  days.  Its  period  seems  to  be  diminishing,  having  been 
about  500  days  when  first  discovered. 


NEW  AND    VAltlABLE  UlAllS.  431 

strument  when  his  telescope  is  pointed  at  the  object.  If  he 
happens  to  niiike  a  mistake  in  writing  down  any  of  these 
numbers — if,  for  exann)le,  he  gets  his  clock-time  one  minute 
or  five  minutes  wrong,  or  i)uts  down  a  wrong  number  of  de- 
grees for  the  position  of  his  circle — he  will  write  down  the 
position  of  the  star  where  none  really  exists.  Then,  some  sub- 
sequent astronomer,  looking  in  this  place  and  seeing  no  star, 
may  think  the  star  has  disappeared,  when,  in  reality,  there  was 
never  any  star  there.  Where  thousands  of  numbers  have  to  be 
written  down,  such  mistakes  will  sometimes  occur;  and  it  is  to 
them  that  some  cases  of  supposed  disappearance  of  stars  are  to 
be  attributed.  There  have,  however,  been  several  cases  of  ap- 
parently new  stars  coming  suddenly  into  view,  of  which  we 
shall  describe  some  of  the  most  renuirkable. 

T  Coroniu :  right  ascension,  15  hours  'A  minutes  IG  seconds;  declination,  26^ 
1G'.;>  N. — This  is  the  "new  star"  wiiieh  bhized  out  in  the  Nortiiein  Crown  in 
ISGf),  as  hereafter  described.  OF  late  years  it  has  remained  between  the  ninth 
and  tenth  magnitudes  without  exiiihitiug  any  remarkable  variations, 

T  Scorpii :  right  ascension,  IG  hours  D  minutes  .'UJ  seconds;  declination,  22^ 
40'.0  8. — Tills  star  was  discovered  by  Auwers,  in  18G(),  in  the  midst  of  a  well- 
known  cluster.  It  gradually  dimiuisiied  during  the  following  months,  and  finally 
disappeared  entirely  among  the  stars  by  which  it  is  surrounded. 

—  Serpentarii :  right  ascension,  17  hours  2;^  minutes  f)  seconds;  declination, 
21°  22'.4  S. — This  is  supposed  to  be  the  celebrated  "new  star"  seen  and  de- 
scribed by  Kepler  in  1G()4,  soon  to  be  described. 

X  Cygni :  right  ascension,  1!)  hours  45  minutes  4G  seconds;  declination,  ;i2'  3G'.0 
N. — This  star  becomes  visii)le  to  the  naked  eye  at  intervals  of  about  4(IG  days,  and 
then  sinks  to  the  twelfth  or  thirteenth  magnitude,  so  that  only  large  telescopes  will 
show  it.     Its  greatest  brigiitness  ranges  from  the  fourth  to  the  sixtli  magnitude. 

1]  A(}uilie  :  right  ascension,  1!)  hours  4G  minutes  G  seconds  ;  declination,  0^ 
41'.2  N. — This  star  varies  from  magnitude  3^  to  4J,  and  is  therefore  one  of 
those  which  can  readily  be  observed  with  the  naked  eye.  Its  period  is  7  days  4 
hours  14  minutes  4  seconds. 

P  Cygni:  right  ascension,  20  hours  liJ  minutes  11  seconds;  declination,  37° 
38'. 7  N. — This  was  supposed  to  be  a  new  star  in  IGOO,  when  it  was  first  seen 
by  Janson.  During  the  remainder  of  the  century  it  varied  from  the  third  to  the 
sixth  magnitude ;  but  during  two  cetituries  which  have  since  elapsed  no  further 
variations  have  been  noticed,  the  star  being  constantly  of  the  fifth  magnitude. 

/u  Cephei :  right  ascension,  21  hours  39  minutes  41  seconds;  declination,  58° 
12'.4  N, — One  of  the  reddest  stars  visil)le  to  the  naked  eye  in  the  northern  hemi- 
sphere. Its  magnitude  is  found  to  vary  from  the  fourth  to  the  fifth  in  a  very  ir- 
regular manner. 


i'32  THE  STELLAR   Uy I  VERSE. 

In  1572  an  apparently  new  Btar  showed  itself  in  Cassiopeia. 
It  was  lirst  seen  by  Tyclio  Uralio  on  November  lltli,  when 
it  had  attained  the  first  niajj^nitiidc.  It  increased  rapidly  in 
brilliancy,  soon  becoming  c({ual  to  Venus,  so  that  good  eyes 
could  discern  it  in  full  daylight.  In  December  it  began  to 
grow  smaller,  and  continued  gradtuilly  to  fade  away  until  the 
following  May,  when  it  disappeared  entirely.  This  was  forty 
years  before  the  invention  of  the  telescope.  Tycho  has  left  us 
an  extended  treatise  on  this  most  remarkable  star. 

Ill  1604:  a  similar  phenomenon  was  seen  in  the  constella- 
tion Ophiuclnis.  The  star  was  first  noticed  in  October  of  that 
year,  when  it  had  attaiiied  the  iirst  magnitude.  In  the  follow- 
ing winter  it  ])egan  to  wane,  but  remained  visible  during  the 
whole  year  1605.  Early  in  1606  it  faded  away  entirely,  hav- 
ing been  visible  for  more  than  a  year.  A  very  full  history  of 
this  star  has  been  left  to  us  by  Kej)ler. 

The  most  striking  recent  case  of  this  kind  was  in  May, 
1866,  when  a  star  of  the  second  magnitude  suddenly  a})peared 
in  Corona  Borealis.  On  the  11th  and  12th  of  that  month  it 
was  remarked  independently  by  at  least  five  observers  in  Eu- 
rope and  America,  one  of  the  first  being  Mr.  Farquhar,  of  the 
United  States  Patent- office.  Whether  it  really  blazed  out  as 
suddenly  as  this  would  indicate  has  uot  been  definitively  set- 
tled. If,  as  M'ould  seeui  most  probable,  it  was  several  days 
attaining  its  greatest  brilliancy,  then  the  only  person  known 
to  have  seen  it  was  Mr.  Benjamin  Ilallowell,  a  well-known 
teacher  near  Washington,  whose  testimony  is  of  such  a  nature 
that  it  is  hard  to  doubt  that  the  star  was  visible  several  days 
before  it  was  generally  known.  On  the  other  hand,  Schmidt, 
of  Athens,  asserts  in  the  most  positive  manner  that  the  star 
was  not  there  on  Mav  10th,  because  he  was  then  scanning 
that  part  of  the  heavens,  and  would  certainly  have  noticed  it. 
However  the  fact  may  have  been  in  this  particular  case,  it  is 
noteworthy  that  none  of  the  new  stars  we  have  described  were 
noticed  until  they  had  nearly  or  quite  attained  their  greatest 
brilliancy,  a  fact  which  gives  color  to  the  view  that  they  have 
all  blazed  up  with  great  rapidity. 


NEW  AND   I'AMABLE  STARH.  433 

In  November,  187G,  a  new  star  of  the  third  magnitude  was 
noticed  by  Schmidt,  of  Athens,  in  the  conetellatiun  Cygnus. 
It  soon  began  to  fade  away,  and  disappeared  from  the  unaided 
Vision  in  a  few  weeks.  The  position  of  the  constellation  Cyg- 
nus becomes  so  unfavorable  fur  observation  in  November  that 
very  few  people  got  a  sight  of  this  object. 

The  view  that  these  bodies  may  l)e  new  creations,  designed 
to  rank  permanently  among  their  fellow-stars,  is  comj)letely 
refuted  l)y  their  transient  character,  if  by  nothing  else.  Their 
apparently  ephemeral  existence  is  in  striking  contrast  to  the 
permanency  of  the  stars  in  general,  which  endure  from  age  to 
age  without  any  change  whatever.  They  are  now  classitied 
by  astronomers  among  the  variable  stars,  their  changes  being 
of  a  very  irregular  and  iitful  character.  There  is  no  serious 
doubt  that  they  were  all  in  the  heavens  as  very  small  stars 
before  they  blazed  forth  in  this  extraordinary  nuinner,  and 
that  they  are  in  the  same  place  yet.  Tiie  position  of  the  star 
of  1572  was  carefully  determined  by  Tycho  lirahe;  and  a 
small  telescopic  star  now  exists  within  V  of  the  place  com- 
puted from  his  observations,  and  is  probably  the  same.  The 
star  of  1S06  was  found  to  have  been  recorded  as  one  of  the 
ninth  magnitude  in  Argelander's  great  catalogue  of  the  starts 
of  the  northern  hemisphere,  completed  several  years  before. 
After  blazing  up  in  the  way  we  have  described,  it  gradually 
faded  away  to  its  former  insignificance,  and  has  shown  no 
further  signs  of  breaking  forth  again.  There  is  a  wide  differ- 
ence between  these  irregular  variations,  or  breaking-forth  of 
light,  on  a  single  occasion  in  the  course  of  centuries,  and  the 
regular  changes  of  Algol  and  /3  Lyra3.  But  the  careful  obser- 
vations of  the  industrious  astronomers  who  have  devoted  them- 
selves to  this  subject  have  resulted  in  the  discovery  of  stars 
of  nearly  every  degree  of  irregularity  between  these  extremes. 
Some  of  them  change  gradually  from  one  magnitude  to  another, 
in  the  course  of  years,  without  seeming  to  follow  any  law  what- 
ever, while  in  others  some  tendency  to  regularity  can  be  faintly 
traced.  The  best  connecting  link  between  new  and  varialjle  stars 
is,  perhaps,  afforded  by  rj  Argus,  which  we  have  just  described. 

29 


434  THE  STELLAR   UNI  FE USE. 

It  is  probable  that  the  variations  of  light  of  which  we  have 
spoken  are  the  result  of  operations  going  on  in  tlie  star  itself, 
which,  it  must  be  renienibered,  is  a  body  of  the  same  order  of 
magnitude  and  brilliancy  with  our  sun,  and  that  these  opera- 
tions are  analogous  to  those  which  produce  the  solar  spots.  It 
was  shown  in  the  chapter  on  the  sun  that  the  i'requency  of 
solar  spots  shows  a  period  of  eleven  years,  during  one  portion 
of  which  there  are  frequently  no  spots  at  all  to  be  seen,  while 
during  another  portion  they  are  very  numerous.  Hence,  if 
an  observer  so  far  away  in  the  stellar  places  as  to  see  our  sun 
like  a  star,  could,  from  time  to  time,  make  exact  measures  of 
tlie  amount  of  light  it  emitted,  he  would  iind  it  to  be  a  vari- 
able star,  with  a  period  of  eleven  years,  the  amount  of  light 
being  least  when  we  see  most  spots,  and  greatest  when  there 
are  few  spots.  The  variation  would,  indeed,  be  so  slight  that 
we  could  not  perceive  it  with  any  photometric  means  which 
we  possess,  but  it  would  exist  nevertheless.  Now,  the  general 
analogies  of  the  universe,  as  well  as  the  testimony  of  the  spec- 
troscope, lead  us  to  believe  that  the  physical  constitution  of 
the  sun  and  the  stars  is  of  the  same  general  nature.  We  may 
therefore  expect  that,  as  we  see  spots  on  the  sun  which  vary 
in  form,  size,  and  number  from  dn  to  day,  so,  if  we  could 
take  a  sufficiently  close  view  of  the  faces  of  the  stars,  we 
should,  at  least  in  some  of  them,  see  similar  spots.  Tt  is  also 
likely  that,  owing  to  the  varying  j)liysical  constitution  of  these 
bodies,  the  number  and  extent  of  the  spots  might  be  found  to 
be  very  different  in  different  stars.  In  the  cases  in  which  the 
spots  covered  the  larger  portion  of  the  surface,  their  variations 
in  number  and  extent  would  alone  cause  the  star  to  vary  in 
light,  from  time  to  ti'Tie.  Finally,  we  have  only  to  suppose 
the  same  kind  of  regularity  which  we  see  in  the  eleven-year 
cycle  of  the  solar  spots,  to  have  a  variation  in  the  brightness 
of  a  star  going  through  a  regular  cycle,  as  in  the  case  of  Algol 
and  Mira  Ceti. 

The  occasional  outbursts  of  stars  which  we  have  described, 
in  wliich  their  light  is  rapidly  increased  a  liundred-fold,  would 
seem  not  to  be  accounted  for  on  the  spot  theory,  without  car- 


NEW  AND   VARIABLE  STARS.  435 

rying  this  theory  to  an  extreme.  It  would,  in  fact,  if  not 
nioditied,  imply  that  ninety-nine  parts  of  the  surface  out  of  a 
hundred  were  ordinarily  covered  with  spots,  and  tliat  on  rare 
occasions  these  spots  all  disappeared.  But  the  spectroscopic 
observations  of  the  star  of  18(50  showed  an  analogy  of  a  little 
different  character  with  operations  going  on  in  our  sun.  Mr. 
lluggins  found  the  spectrum  of  this  star  to  be  a  continuous 
one,  crossed  by  bright  lines,  the  position  of  which  indicated 
that  they  proceeded  partly  or  wholly  from  glowing  hydrogen. 
Tlie  continuous  spectrum  was  also  crossed  'n'  dark  absorption 
lines,  indicating  that  the  light  had  passed  through  an  atmos- 
phere of  comparatively  cool  gas.  Mr.  llnggins's  interpreta- 
tion of  this  is  that  there  was  a  sudden  and  extraordinary  out- 
burst of  hydrogen  gas  from  the  star  which,  by  its  own  light, 
as  well  as  by  heating  up  the  whole  surface  of  the  star,  caused 
tlie  immense  accession  of  brilliancy.  Now,  w^e  have  shown 
that  the  red  flames  seen  around  the  sun  during  a  total  eclipse 
are  caused  by  eruptions  of  hydrogen  from  his  interior ;  more- 
over, these  eru})tions  are  generally  connected  with  fa(!u]a?,  or 
portions  of  the  sun^s  disk  several  times  more  brilliant  tlian  the 
rest  of  the  pliotosjthere.  Hence,  it  is  not  unliliely  that  the 
blazing-forth  of  this  star  arose  from  an  action  similar  to  that 
which  produces  the  solar  flames,  only  on  an  immensely  larger 
scale. 

We  have  thus  in  the  spots,  fa(!ula>,  and  protuberances  of 
the  sun  a  few  suggestions  as  to  what  is  }>robably  going  on  in 
those  stars  which  exhibit  the  extraordinary  changes  of  light 
which  we  have  described.  Is  there  any  possibility  that  our 
sun  may  be  subject  to  such  outbursts  of  light  and  iieat  as 
those  we  !iave  described  in  the  cases  of  apparemlj'  new  and 
temporary  stars?  We  may  almost  say  that  the  continued  ex- 
istence of  the  human  race  is  involved  in  this  question  ;  for  if 
the  heat  of  the  sun  should,  even  for  a  few  days  only,  be  in- 
creased a  hundred-fold,  the  hiiijher  orders  of  animal  and  veer- 
etable  life  would  be  destroyed.  We  can  only  reply  to  it  that 
the  general  analogies  of  nature  lead  us  to  believe  that  we 
need  not  feel  any  apprehension  of  such  a  catastrophe.     Not 


4:36  THE  STELLAR   UNIVERSE. 

the  sliijlitest  certain  variation  of  the  solar  heat  has  been  de- 
tected  since  the  invention  of  the  tlierniometer,  and  the  gen- 
eral constancy  of  the  light  emitted  by  ninety-nine  stars  out  of 
every  hundred  may  inspire  ns  with  entire  confidence  that  no 
sudden  and  destructive  variation  need  be  feared  in  the  case 
of  our  sun. 

§  4.  Double  Stars. 

Telescopic  examination  shows  that  many  stars  which  seem 
single  to  the  naked  eye  are  really  double,  or  coinposed  of  a 
pair  of  stars  lying  side  by  side.  There  are  in  the  heavens 
several  pairs  of  stars  the  components  of  which  are  so  close 
together  that,  to  the  naked  eye,  they  seem  almost  to  touch 
each  other.  One  of  the  easiest  and  most  beautiful  of  these 
is  in  Taurus,  quite  near  Aldebaran.  Here  the  two  stars  6' 
Tauri  and  6/"  Tauri  are  each  of  the  fourth  magnitude.  An- 
other such  pair  is  a  Capricorni,  in  which  the  two  pairs  are  un- 
equal. Here  an  ordinary  eye  has  to  look  pretty  carefully  to 
see  the  smaller  star.  Yet  another  pair  is  e  Lyra},  the  com- 
ponents of  which  are  so  close  that  only  a  good  eye  can  dis- 
tinguish them.  These  pairs,  however,  are  not  considered  as 
double  stars  in  astronomy,  because,  although  to  the  naked  e^'e 
they  seem  so  close,  yet,  when  viewed  in  a  telescope  of  high 
power,  they  are  so  wide  apart  that  they  cannot  be  seen  at  the 
same  time.  The  telescopic  double  stars  are  formed  of  com- 
ponents only  a  few  seconds  apart;  indeed,  in  many  cases, only 
a  fraction  of  a  second.  The  large  majority  of  those  which 
are  catalogued  as  doubles  range  from  half  a  second  to  fifteen 
seconds  in  distance.  When  they  exceed  the  latter  limit,  th.ey 
are  no  longer  objects  of  special  interest,  because  they  may 
be  really  witiiout  any  connection,  and  appear  together  only 
because  they  lie  in  nearly  the  satne  straight  line  from  our 
system. 

The  most  obvious  qncstion  which  suggests  itself  here  is 
whether  in  any  case  there  is  any  real  connection  between  the 
two  stars  of  the  pair,  or  whether  they  do  not  appear  close  to- 
gether, simply  because  they  chance  to  lie  on  nearly  the  same 


DOUBLE  STARS.  437 

straight  line  from  the  earth.  Tliat  some  stars  do  appear  dou- 
ble ill  this  way  there  is  no  doubt,  and  such  pairs  are  called 
"optically  double."  But  notwithstanding  the  innnense  num- 
ber of  visible  stars,  the  chance  of  many  pairs  falling  within 
a  few  seconds  of  each  other  is  quite  small ;  and  the  number 
of  close  double  stars  is  so  great  as  to  preclude  all  possibility 
that  they  appear  together  only  by  chance.  If  any  further 
proof  was  wanted  that  the  stars  of  these  pairs  are  really  i>hys- 
ically  connected,  and  therefore  close  together  in  reality  as  well 
as  in  appearance,  it  is  fonnd  in  the  fact  that  many  of  them 
constitute  systems  in  which  one  revolves  round  the  other,  or, 
to  si)eak  more  exactly,  in  which  each  revolves  round  the  cen- 
tre of  gravity  of  the  jiair.  Such  pairs  are  called  hinanj  si/s- 
terns,  to  distinguish  them  from  those  in  which  no  such  revolu- 
tion has  been  observed.  The  revolution  of  these  binary  sys- 
tems is  generally  very  slow,  requiring  many  centuries  for  its 
acconq)lishment ;  and  the  slower  the  motion,  the  longer  it 
will  take  to  perceive  and  determine  it.  Generally  it  has  been 
detected  by  astronomers  of  one  generation  conqjaring  their 
observations  with  those  of  their  predecessors ;  for  instance, 
when  the  elder  Struve  compared  his  observations  with  those 
of  Ilerschel,  and  when  Dawes  or  the  younger  Struve  compared 
with  the  elder  Struve,  a  great  number  of  pairs  were  found  to 
be  binary.  As  every  observer  is  constantly  detecting  nev/ 
cases  of  motion,  the  number  of  binary  systems  known  to  as- 
tronomers is  constantly  increasinn:. 

A  brief  account  of  the  manner  in  which  these  objects  are 
measured  may  not  be  out  of  place.  For  the  purpose  in  ques- 
tion, the  eye-piece  of  the  telescope  must  be  provided  with  a 
"filar  micrometer,"  the  inq3ortant  part  of  which  consists  of  a 
pair  of  parallel  spider-lines,  one  of  which  can  be  moved  side- 
ways by  a  very  tine  screw,  and  can  thus  l)e  made  to  jiass  back 
and  forth  over  the  other.  The  exact  distance  apart  of  the 
lines  can  be  determined  from  the  position  of  the  screw.  Tiie 
wliole  micrometer  turns  round  on  an  axis  parallel  to  the  tel- 
escope, the  centre  of  which  is  in  tlio  centre  of  the  field  of 
view.    To  get  the  direction  of  one  star  from  the  other,  the  ob- 


438 


THE  STELLAR   UNIVERSE. 


server  turns  the  micrometer  round  until  the  spider-lines  are 
parallel  to  the  line  joining  the  two  stars,  as  shown  in  Fig.  98, 
and  he  then  reads  the  position  circle.  Knowing  what  the 
position  circle  reads  when  he  turns  the  wires  so  that  the  star 
shall  run  along  them  by  its  diurnal  motion,  the  difference  of 
the  two  angles  shows  the  angle  which  the  line  joining  the 
two  stars  makes  with  the  celestial  parallel.  To  obtain  the 
distance  apart  of  the  stars,  the  observer  turns  the  micrometer 
00°  from  the  position  in  Fig.  98,  and  then  turns  the  screw  and 
moves  the  telescope,  until  each  star  is  bisected  by  one  of  the 
wires,  as  shown  in  Fig.  99.  The  position  of  the  wires  is  then 
interchanged,  and  the  measure  is  repeated.      The  mode  in 

N 
P 


S 


l'"i(j.9S.  Fio.  99.  Fii      •  ./. 

wliich  the  direction  of  one  star  from  another  is  reconed  is 
this:  Imagine  a  line,  SN,  in  Fig.  100,  drawn  due  north  from 
the  brighter  star,  and  another,  *V/-*,  drawn  tin-ough  the  smaller 
star.  Then  the  angle  NSP  which  these  two  lines  nuike  with 
each  other,  counted  from  north  towards  cast,  is  the  position 
angle  of  the  stars,  the  changes  in  wliich  show  the  revolution 
of  one  star  around  the  other. 

In  a  few  of  the  binary  systems  the  period  is  so  short  that 
a  complete  revolution,  or  more,  of  the  two  stars  round  each 
other  has  been  observed.  As  a  general  rule,  the  pairs  which 
have  the  most  rapid  motion  are  very  close,  and  therefore  of 
comparatively  recent  discovery,  and  difticuU  to  observe.  One 
or  two  are  suspected  to  have  a  period  of  \vy>  than  thirty  years, 
but  tliey  are  very  hard  to  measure. 

Binanj  Systems  of  ^Short  Period. — The  following  table  shows 


DOUBLE  STAIiS.  439 

the  periods  of  revolution  in  the  case  of  those  stars  which  liave 
been  observed  through  a  complete  revolution,  or  of  which  the 
periods  have  been  well  determined : 


42  Comaj 2(5  years. 

5  Herciilis ;{")      " 

Stnive,  :{1L>1  40      " 

j;  Coronaj 40      " 

Sirius r)0      " 

K  Caiicri 58      " 


?  Ursa;  Majoiis 01?  years. 

»;  (^oroiiai  Burealis (JT      '* 

n  Centauri 77      " 

H  Ophiiichi 1)2      " 

\  Opliiuchi <)(•>      " 

£,  Scorpii 98      " 


Two  or  three  others  arc  suspected  to  move  very  rapidly,  but 
tliey  are  so  very  close  and  difficult  that  it  is  only  on  favora- 
ble occasions  that  they  can  be  seen  to  be  double.  '  One  of 
the  most  remarkable  stars  in  this  list  is  Sirius,  the  period  of 
which  is  calculated,  not  from  the  observations  of  the  satel- 
lite, but  from  the  motion  of  Sirius  itself.  It  has  long  been 
known  that  the  proper  motion  of  this  star  is  subject  to  cer- 
tain periodic  valuations ;  and,  on  investigating  these  varia- 
tions, it  was  found  by  Peters  and  Auwers  that  they  could  be 
completely  represented  by  supposing  that  a  satellite  was  re- 
volving around  the  planet  in  a  certain  orbit.  The  elements 
of  this  orbit  were  all  determined  except  the  distance  of  the 
satellite,  M'hich  did  not  admit  of  determination.  Its  direction 
could,  however,  be  computed  from  time  to  time  almost  us  ac- 
curately as  if  it  wei'e  actually  seen  with  the  telescope.  But, 
before  the  time  of  which  we  speak,  no  one  had  ever  seen  it. 
Indeed,  although  many  observers  must  have  examined  Sirius 
from  time  to  time  with  good  telescopes,  it  is  not  likely  that 
they  made  a  careful  search  in  the  predicted  direction. 

Such  was  the  state  of  the  question  until  February,  1S02, 
when  Messrs.  Alvan  Clark  &  Sons,  of  Cambridgeport,  were 
completing  their  eighteen-inch  glass  for  the  Chicago  Observa- 
tor3\  Turning  the  glass  one  evening  on  Sirius,  for  the  pur- 
pose of  trying  it,  the  practised  eye  of  the  younger  Clark  soon 
detected  something  unusual.  "  Why,  father,"  he  exclaimed, 
"  the  star  has  a  companion  !"  The  father  looked,  and  there 
was  a  faint  companion  due  east  froin  the  bright  star,  and  dis 
taut  about  10".     This  was  exactly  the  predicted  direction  for 


440  THE  STELLAR   UNIVERSE. 

that  time,  tliongli  the  discoverers  knew  nothing  of  it.  As  tlie 
news  went  round  tlie  world,  all  the  great  telescopes  were 
pointed  on  Sirius,  and  it  was  now  found  that  when  ohservers 
knew  where  the  companion  was,  many  telescopes  would  show 
it.  It  lay  in  the  exact  direction  which  theory  had  predicted 
for  that  time,  and  it  was  now  observed  with  the  greatest  inter- 
est, in  order  to  see  whether  it  was  moving  in  the  direction  of  the 
theoretical  satellite.  Four  years'  observation  showed  that  this 
was  reall}'  the  case,  so  that  hardly  any  doubt  could  remain  that 
this  almost  invisible  object  was  really  the  body  which,  by  its  at- 
traction and  revolution  around  Sirius,  had  caused  the  inequal- 
ity in  its  motion.  At  the  same  time,  the  correspondence  has 
not  since  proved  exact,  the  observed  companion  having  moved 
about  half  a  degree  per  aimum  more  rapidly  than  the  theo- 
retical one.  This  difference,  though  larger  than  was  expected, 
is  probably  due  to  the  inevitable  errors  of  the  very  delicate 
and  difficult  ol)servations  from  which  the  movements  of  the 
theoretical  companion  were  computed. 

The  visibility  of  this  very  in'  resting  and  difficult  object 
depends  almost  as  much  on  the  altitude  of  Sirius  and  the  state 
of  the  atmosphere  as  on  the  power  of  the  telescope.  When 
the  images  of  the  stars  are  very  bad,  it  cannot  be  seen  even 
in  the  great  Washington  telescope,  while  there  are  cases  of  its 
being  seen  under  extraordinarily  favorable  conditions  with  tel- 
escopes of  six  inches  aperture  or  less.  These  favorable  condi- 
tions are  indicated  to  the  naked  eye  by  the  absence  of  twinkling. 

A  case  of  the  same  kind, except  that  the  disturbing  satellite 
lias  not  been  seen,  is  found  in  Procyon.  Bessel  long  ago  sus- 
pected that  the  position  of  this  star  was  changed  by  some  at- 
tracting body  in  its  neighborhood,  but  he  did  not  reach  a  defi- 
nite conclusion  on  the  subject.  Auwers,  having  made  a  care- 
ful investigation  of  all  the  observations  since  the  time  of  Bi-ad- 
ley,  found  that  the  star  moved  around  an  invisible  centre  1" 
distant,  which  was  probably  the  centre  of  gravity  of  the  star 
and  an  invisible  satellite.  This  satellite  has  been  carefully 
searched  for  with  great  telesco})es  during  the  last  few  years, 
but  without  success. 


CLVSTEIiS  OF  STABS.  441 

Triple  and  Multiple  Stars.  —  Besides  double  stars,  groups 
of  three  or  more  stars  are  frequently  found.  Such  objects 
are  known  as  triple,  quadruple,  etc.  They  commonly  occur 
through  one  of  the  stars  of  a  wide  pair  being  itself  a  close 
double  star,  and  very  often  the  duplicity  of  the  component 
has  not  been  discovered  till  long  after  it  was  known  to  form 
one  star  of  a  pair.  For  instance,  ju  Ilerculis  was  recognized 
as  a  double  star  by  Sir  W.  Ilerschel,  the  companion  star  being 
about  30"  distant,  and  much  smaller  than  fi  itself.  In  18.56, 
Mr.  Alvan  Clark,  trying  one  of  his  glasses  npon  it,  found  that 
the  small  companion  was  itself  double,  being  composed  of  two 
nearly  equal  stai-s,  about  1"  apart.  This  close  pair  proves  to 
be  a  binary  system  of  short  period,  more  than  half  a  revolu- 
tion of  the  two  stars  around  each  other  having  been  made 
since  1856.  Another  case  of  the  same  kind  is  y  Andromedii?, 
which  was  found  by  Ilerschel  to  have  a  companion  about  10'' 
distant,  while  Struve  found  this  companion  to  be  itself  double. 

Many  double  and  multiple  stars  are  interesting  objects  for 
telescopic  examination.  We  give  in  the  Appendix  a  list  of 
the  more  interesting  or  remarkable  of  them. 

§  5.  Clusters  of  Stars. 

A  very  little  observation  with  the  telescope  will  show  th  u 
while  the  brighter  stars  are  scattered  nearly  equally  over  tlie 
whole  celestial  vault,  this  is  not  the  case  with  the  smaller  ones. 
A  number  of  stars  which  it  is  not  possible  to  estimate  are 
found  to  be  aggregated  into  clusters,  in  which  the  separate 
stare  are  so  small  and  so  numerous  that,  with  insufficient  tele- 
scopic power,  they  present  the  appearance  of  a  mass  of  clcudy 
light.  We  find  clusters  of  every  degree  of  aggregation.  At 
one  extreme  we  may  place  the  Pleiades,  or  "seven  stars" 
which  form  so  well-known  an  object  in  our  winter  sky,  in 
which,  however,  only  six  of  the  stars  are  plainly  visible  to  the 
naked  eye.  There  is  an  old  myth  that  this  group  originally 
consisted  of  seven  stars,  one  of  which  disappeared  from  the 
heavens,  leaving  but  six.  But  a  very  good  eye  can  even  now 
see  eleven  when  the  air  is  clear,  and  the  telescope  shows  from 


442  THE  STELLAR   UNIVERSE. 

fifty  to  a  liundred  more,  according  to  its  power.    "We  present  a 
view  of  this  group  as  it  appears  through  a  small  telescope. 

No  absolute  dividinc;-line  can  be  drawn  between  such  wide- 
ly  extended  groups  as  the  Pleiades  and  the  densest  clusters. 


Fia.  101. — Teleson|)ic  view  of  the  Pleiiides,  after  Eiicelininin.  The  six  larirer  stars  nre  those 
easily  seen  by  ordinary  eyes  without  a  telescope,  wliile  the  four  ijext  in  size,  havinL' 
four  rays  each,  can  be  seen  l)y  very  good  eyes.  About  an  inch  from  the  upper  right- 
haud  corner  is  a  pair  of  Hinall  etar.s  which  a  very  keen  eye  can  see  as  a  single  star. 

The  cluster  Praesepe,  in  the  constellation  Cancer  (Map  III., 
right  ascension,  8  hours  20  minutes;  declination,  20°  10'  N.), 
is  plainly  visible  to  the  naked  eye  on  a  clear,  moonless  night, 
as  a  nebulous  mass  of  light.     Examined  with  a  small  tele- 


CLUSTERS  OF  STARS.  443 

scope,  it  is  found  to  consist  of  a  group  of  stars,  ranging  fi-oin 
tlio  seventli  or  eighth  magnitude  upwards.  For  examination 
witli  a  small  telescope,  one  of  the  most  beautiful  groups  is  in 
the  constellation  Perseus  (Map  I.,  right  ascension,  2  hours  10 
minutes  ;  declination,  57°  N.).  It  is  seen  to  the  best  advantage 
with  a  low  magnifying  power,  between  twenty-five  and  fifty 
times,  and  may  easily  be  recognized  by  the  naked  eye  as  a 
little  patch  of  light. 

The  heavens  afford  no  objects  of  more  interest  to  the  con- 
templative mind  than  some  of  these  clusters.  Many  of  them 
are  so  distant  that  the  most  powerful  telescopes  ever  made 
show  tlicm  only  as  a  patch  of  star-dust,  or  a  nuvss  of  light  so 
faint  that  the  separate  stars  cannot  be  distinguished.  Their 
distance  from  ns  is  such  that  they  are  beyond,  not  only  all 
our  means  of  measurement,  but  all  our  powers  of  estimation. 
Minute  as  they  appear,  there  is  nothing  that  we  know  of  to 
prevent  our  supposing  each  of  them  to  be  the  centre  of  a 
group  of  planets  as  extensive  as  our  own,  and  each  planet  to 
be  as  full  of  inhabitants  as  this  one.  We  may  thus  think  of 
them  as  little  colonies  on  the  outskirts  of  creation  itself,  and 
as  we  see  all  the  suns  whicli  ijive  them  lii>:ht  condensed  into 
one  little  speck,  we  might  be  led  to  think  of  the  inhabitants 
of  the  various  svstems  as  holdini;;  intercourse  with  each  other. 
Yet,  were  we  transported  to  one  of  these  distant  clusters,  and 
stationed  on  a  ])lanet  circling  one  of  the  suns  which  compose 
it,  instead  of  finding  the  neighl)oring  suns  in  close  proximity, 
we  should  only  see  a  firmament  of  stars  around  us,  such  as  we 
see  from  the  earth.  Probably  it  would  be  a  brighter  firma- 
ment, in  which  so  many  stars  would  glow  witli  more  than  the 
S})lcndor  of  Sirius,  as  to  make  the  night  far  brighter  than 
ours;  but  the  inhabitants  of  the  neighboring  worlds  would  as 
completely  elude  telescopic  vision  as  the  inhabitants  of  Mars 
do  here.  Consequently,  to  the  inhabitants  of  every  planet  in 
the  cluster,  the  question  of  the  plurality  of  worlds  might  be 
as  insolvable  as  it  is  to  us. 

To  give  the  reader  an  idea  what  the  more  di!=tant  of  these 
star  clusters  looks  like,  we  present  two  views  from  Sir  John 


444 


THE  STELLAR   UN  I  VERS. 


Ilerscliers  observations  at  the  Capo  of  Good  Hope.  Fig.  102 
shows  the  duster  numbered  2322  in  llerechel's  catalogue,  and 
known  as  47  Toucani.  Tiiat  astronomer  describes  it  as  ''a 
most  glorious  globular  cluster,  the  stars  of  the  fourteenth  mag- 
nitude immensely  numerous.  It  is  compressed  to  a  blaze  of 
light  at  the  centre,  the  diameter  of  the  more  compressed  ])art 
being  30"  in  right  ascension."  Fig.  103  is  No.  3504  of  ller- 
schel :  "  The  noble  globular  cluster  a>  Centauri,  beyond  all 
comparison  the  richest  and  largest  object  of  the  kind  in  the 
heavens.      The  stars  are  literally  innumerable,  and  as  their 


South 


North. 

Fig.  102.— Cluster 47 Toncani.    Rijjht ascen- 
sion, 0  hours  IS  niiuutes;   declination, 


Fig.  103.— Cluster  u  Centauri.    TJiirtit  iiscen- 
6ion,  13  houru  20  niiuuteH ;  decliuatiou, 


12°  45'  S.  46°  52'  S. 

total  light  when  received  by  the  naked  eye  affects  it  hardly 
more  than  a  star  of  the  fifth  or  fourth  to  fifth  magnitude,  the 
minuteness  of  each  star  may  be  imagined." 

§  6.  Nehulce. 

Kebulre  appear  to  us  as  masses  of  soft  diffused  light,  of 
greater  or  less  extent.  Generally  these  masses  are  very  ir- 
regular in  outline,  but  a  few  of  them  are  round  and  well- 
defined.  These  are  termed  plandar^j  nebulai.  It  may  some- 
times be  impossible  to  distinguish  between  star  clusters  and 
nebulae,  because  when  the  power  of  the  telescope  is  so  low 
that  the  separate  stars  of  a  cluster  cannot  be  distinguished, 
they  will  present  the  appearance  of  a  nebula.  To  the  naked 
eye  the  cluster  Pra3sepe,  described  in  the  last  chapter,  looks 


NEBULJi.  445 

exactly  like  a  nebula,  though  a  very  small  telescope  will  re- 
solve vit  into  stars.  The  early  observers  with  telescopes  de- 
scribed many  objects  as  nebulai  which  the  more  powerful  in- 
strtnnents  of  Ilerschel  showed  to  be  clusters  of  stars.  Thus 
arose  the  two  classes  of  resolvable  and  irresolvable  nebula?, 
the  lirst  comprising  such  as  could  be  resolved  into  stars,  and 
the  second  such  as  could  not.  It  is  evident,  from  what  we 
have  just  said,  that  this  distinction  would  depend  partly  on 
the  telescope,  since  a  nebula  which  was  irresolvable  in  one 
telescope  nu'ght  be  resolvable  in  another  telescope  of  greater 
power.  This  suggests  the  question  whether  all  nebuliii  nuiy 
not  really  be  clusters  of  stars,  those  which  are  irresolvable  ap- 
pearing so  merely  because  their  distance  is  so  great  that  the 
separate  stars  which  compose  them  cannot  be  distinguished 
with  our  most  powerful  telescopes.  If  this  were  so,  there 
would  be  no  such  thing  as  a  real  nebula,  and  everything 
which  appears  as  such  should  be  classified  as  a  star  cluster. 
The  spectroscope,  as  we  shall  presently  show,  has  settled  this 
question,  by  showing  that  many  of  these  objects  are  immense 
masses  of  glowing  gas,  and  therefore  cannot  be  stars. 

Classification  and  Forms  of  Nehuloi. — The  one  object  of  this 
class  which,  more  than  all  others,  has  occupied  the  attention 
of  astronomers  and  excited  the  wonder  of  observers,  is  the 
great  nebula  of  Orion.  It  surrounds  the  middle  of  the  three 
stars  which  form  the  sword  of  Orion.  Its  position  may  bo 
found  on  Maps  II.  and  III.,  in  right  ascension  5  hours  28 
minutes,  declination  6°  S.  A  good  eye  will  perceive  that 
this  star,  instead  of  looking  like  a  bright  point,  as  the  other 
stars  do,  has  an  ill-defined,  hazy  appearance,  due  to  the  sur- 
rounding nebula?.  This  object  was  first  described  by  Iluy- 
ghens  in  1659,  as  follows  : 

"  There  is  one  phenomenon  among  the  fixed  stars  worthy 
of  mention  which,  so  far  as  I  know,  has  hitherto  been  noticed 
by  no  one,  and  indeed  cannot  be  well  observed  except  with 
large  telescopes.  In  the  sword  of  Orion  are  three  stars  quite 
close  together.  In  1656,  as  I  chanced  to  be  viewing  the  mid- 
dle one  of  these  with  the  telescope,  instead  of  a  single  star. 


44G 


THE  STELLAR   VNIVERSE. 


twelve  sliowed  themselves  (a  not  uncommon  circumstance). 
Three  of  tliese  ahuost  touched  each  other,  and,  vith  four  oth- 
ers, shone  through  a  nebula,  so  that  the  space  around  them 
seemed  far  brighter  than  the  rest  of  the  heavens,  which  was 
entirely  clear,  and  appeared  quite  black,  the  effect  being  that 
of  an  opening  in  the  sky,  through  which  a  brighter  region 
was  visible."* 


Fig.  104.— The  gre.it  nebula  of  Orion,  ii«  driiwii  by  Triiuvelot  with  the  tweuty-sis-iuch 

Washington  telescope. 

Since  tliat  time  it  has  been  studied  with  large  telescopes 
by  a  great  number  of  observers,  including  Messier,  the  two 


*  Si/stema  Saturnium,  p.  8.  The  last  remiirk  of  Iluyghens  seems  to  have  pro- 
duced the  impression  that  he  or  some  of  the  early  observers  considered  the  nebula; 
to  be  real  openings  in  the  firmament,  through  which  they  got  glimpses  of  the 
glory  of  the  empyrean.  But  it  may  be  doubted  whether  the  old  idea."  of  the  firma- 
ment and  the  empyrean  were  entertained  by  any  astronomer  after  the  invention 
of  tiie  telescope,  and  there  is  nothing  in  the  remark  of  Iluyghens  to  indicate  that 
he  thought  the  opening  really  existed.     His  words  are  rather  obscme. 


NEBUL.E.  447 

Ilerscliels,  Rosse,  Stnivo,  and  tlie  Bonds.  Tbe  representation 
wliich  we  give  in  Fig.  104  is  from  a  draving  made  by  Mr. 
Trouvelot  witli  tlie  great  Washington  telescope.  In  brilliancy 
and  vai'iety  of  detail  it  exceeds  any  oilier  nebula  vi.<ible  in 
the  northern  hemisphere.  The  central  point  of  interest  is  oc- 
cupied by  four  comparatively  bright  stars,  easily  distinguished 
by  a  small  telescope  with  a  magnifying  power  of  40  or  50, 
combined  with  two  small  ones,  requiring  a  nine-inch  telescope 
to  be  well  seen.  The  whole  of  these  form  a  sextuple  group, 
included  in  a  space  a  few  seconds  square,  which  alone  would 
be  an  interesting  and  remarkable  object.  Besides  these,  the 
nebula  is  dotted  with  so  many  stars  that  they  would  almost 
constitute  a  cluster  by  themselves. 

In  the  winter  of  lS64-'05,  the  spectrum  of  this  object  was 
examined  iiidependently  by  Secchi  and  Iluggins,  who  found 
that  it  consisted  of  three  bright  lines,  and  hence  concluded 
that  the  nebula  was  composed,  not  of  stars,  but  of  glowing- 
gas.  The  position  of  one  of  the  lines  was  near  that  of  a  line 
of  nitrogen,  while  another  seemed  to  coincide  with  a  hydrogen 
line.  There  is,  therefore,  a  certain  })robability  that  this  object 
is  a  mixture  of  hydrogen  and  nitrogen  gas,  though  this  is  a 
point  on  which  it  is  impossible  to  speak  with  certainty. 

Another  brilliant  nebula  visible  to  the  naked  eye  is  the 
great  one  of  Andromeda  (Maps  II.  and  V.,  right  ascension, 
0  hours  35  minutes  ;  declination,  40°  N.).  The  observer  can 
see  at  a  glance  M'ith  the  naked  eye  that  this  is  not  a  star,  but 
a  mass  of  diffused  light.  Indeed,  untrained  observers  have 
sometimes  very  natiu'ally  mistaken  it  for  a  comet.*  It  was 
first  described  by  Marius,  in  1614,  who  conqx  'od  its  light  to 
that  of  a  candle  shining  through  horn.  This  gives  a  very 
good  idea  of  the  singular  imprcision  it  produces,  which  is  that 
of  an  object  not  self-luminous,  but  translucent,  and  illuminated 
by  a  very  brilliant  light  behind  it.     With  a  small  telescope,  it 

*  A  sliip-cnptain  who  had  crossed  the  Athintic  once  visited  the  Cnmhridgc  Oh- 
seivrttory,  to  tell  Professor  Bond  that  he  had  seen  a  small  comet,  which  retnained 
ill  sight  during  his  entire  voyage.  The  ohject  proved  to  be  the  nebula  of  An- 
dromeda. 


448  THE  STELLAR   UNIVERSE. 

is  easy  to  imagine  it  to  be  a  solid  like  honi ;  but  with  a  large 
one,  the  effect  is  much  more  that  of  a  great  mass  of  matter, 
like  fog  or  mist,  which  scatters  and  reflects-  lie  light  of  a  brill- 
iant body  in  its  midst.  That  this  impression  can  be  correct, 
it  would  be  hazardous  to  assert ;  but  the  result  of  a  spectrum 


Pio.  105.— The  annular  nebula  in  Lyra.    Drawn  by  Professor  E.  S.  Ilolden. 

analysis  of  the  light  of  the  nel)ula  certainly  seems  to  favor  it. 
Unlike  most  of  tiio  nebula?,  its  spectrum  is  a  continuous  one, 
simikr  to  the  ordinary  spectra  from  lieated  bodies,  thus  indi- 
catii  g  tiiat  the  light  emanates,  not  from  a  glowing  gas,  b'.  '; 
from  matter  in  the  solid  or  lit^uid  state.     This  would  suggest 


NEBULJ^.  449 

the  idea  that  the  object  is  really  an  immense  star- cluster,  so 
distant  that  the  most  powerful  telescopes  cannot  resolve  it. 
Though  we  cannot  positively  deny  the  possibility  of  this,  yet 
in  the  most  powerful  telescopes  the  light  fades  away  so  softly 
and  gradually  that  no  such  thing  as  a  resolution  into  stars 
seems  possible.  Indeed,  it  looks  less  resolvable  and  more  like 
a  gas  in  the  largest  telescopes  than  in  those  of  moderate  size. 
If  it  .s  redly  a  gas,  and  if  the  spectrum  is  continuous  through- 
out the  whole  extent  of  the  nebula,  it  would  indicate  either 
that  **"  shone  by  reflected  light,  or  that  the  gas  was  subjected 
to  a  great  pressure  almost  to  its  outer  limit,  which  hardly  seems 
possible.  But,  granting  that  the  light  is  reflected,  we  cannot 
say  whether  it  originates  in  a  single  bright  star  or  in  a  num- 
ber of  small  ones  scattered  about  through  the  nebula. 

Another  extraordinary  object  of  this  class  is  tiie  annular,  or 
ring-nebula  of  Lyra,  situated  in  that  constellation,  about  half- 
way between  the  stars  j3  and  j.  In  the  older  telescopes  it 
looked  like  a  perfect  ring;  but  the  larger  ones  of  modern  times 
show  that  the  opening  of  the  ring  is  really  filled  with  nebu- 
lous light ;  in  fact,  that  we  have  here  an  object  of  very  regular 
outline,  in  which  the  outer  portion  is  brighter  tlian  the  inte- 
rior. Its  form  is  neither  cii'cular  nor  exactly  elliptic,  but  egg- 
sha})ed,  one  end  being  more  pointed  than  the  other.  A  mod- 
e.'ate-sized  telescope  will  show  it,  but  a  large  one  is  required 
to  see  it  to  good  advantage. 

It  would  appear,  from  a  comparison  of  drawings  made  at 
different  dates,  that  some  nebuhi^  are  subject  to  great  changes 
of  form.  Especially  does  this  hold  true  of  the  nebula  siu*- 
rounding  the  remarkable  variable  star  ij  Argus.  In  many 
other  nebuljE  changes  have  been  suspected ;  but  the  softness 
and  indistinctness  of  outline  which  characterize  most  of  these 
objects,  and  the  great  difference  of  their  aspect  when  seen  in 
telescopes  of  very  different  powers,  make  it  difKcult  to  prove  a 
change  from  mere  differences  of  drawing.  One  of  the  strong- 
est cases  in  favor  of  change  has  been  made  out  by  Professor 
llolden  from  a  study  of  drawings  and  descriptions  of  what  is 
called  the  "  Omega  nebula,"  from  a  resemblance  of  one  of 

30 


450 


THE  STELLAR    LNIVEIISE. 


FiQ.  106.— The  Omega  nebula;  Hei'scliel  2008.      Right  nsceneioii,  IS  hours  13  miuules; 
decliiiution,  16"  U'  S.    After  lloldeu  and  Trouvelot. 

its  branches  to  tlie  Greek  letter  II.  We  present  a  figure  of 
this  object  as  it  now  appears,  from  a  drawing  by  Professor 
iloldeu  and  Mr.  Trouvelot,  with  the  great  Washington  tele- 
scope. It  is  the  branch  on  the  left-hand  end  of  the  nebula 
which  was  formerly  suj^posed  to  have  tlie  form  of  il. 

As  illustrative  of  the  fantastic  forms  which  nebulae  some- 
times assume,  we  present  Ilerschel's  views  of  two  more  neb- 
ulae. Tliat  shown  in  Fig.  108  he  calls  the  "  looped  nebula," 
and  describes  as  one  of  the  most  extraordinary  objects  in  the 
heavens.  It  cannot  be  seen  to  advantage  except  in  the  south- 
ern hemisphere. 

Distrihuimi  of  the  Nehuhv.  —  A  remarkable  feature  of  the 
distribution  of  the  nebuhe  is  that  they  are  most  numerous 
wh'U'e  the  stars  are  least  so.  While  the  stars  grow  thicker  as 
we  approacli  the  region  of  tlie  ]\[ilky  Wa}^  the  nebulae  dimin- 
ish in  number.     Sir  Jolm  llerschel  remarks  that  one-third  of 


NEBULJS. 


451 


Fio.  107. — Nebula  Ilerschel  3722.    Rijjht  asce'..8ion,  17  hours  56  miuutes;  declination,  24* 

21'  S.     Viler  Sir  Jolin  Ilerschel. 

the  nebulous  contents  of  the  heavens  are  congrcijated  in  a 
broad,  irregidar  })atch  occupying  about  one-eiglith  the  sur- 
face of  the  celestial  sphere,  extending  from  Ursa  Major  in  the 
north  to  Virgo  in  the  south.  If,  however,  we  consider,  not  the 
true  nebulaj,  but  sti.v  roisters,  we  find  the  same  tendency  to 
condensation  in  the  Milky  Way  that  we  do  in  the  stars.  We 
thus  have  a  clearly  marked  dis- 
tinction between  nebulae  and 
stars  as  regards  the  law  of  their 
distribution.  The  law  in  ques- 
tion can  be  most  easily  under- 
stood by  the  non-mathematical 
reader  by  supposing  the  starry 
sphere  in  such  a  position  that 
the  Milky  Way  coincides  with 
the  horizon.  Then  the  stars  and 
star  clusters  will  be  fewest  at  the 
zenith,  and  will  increase  in  number  as  we  approach  the  horizon. 
Also,  in  the  invisible  hemisphere  the  same  law  will  hold,  the 
stars  and  clusters  being  fewest  under  our  feet,  and  will  increase 
as  we  approach  the  horizon.     But  the  true  nebuhi3  will  then 


Fio.  lOS.— The  looped  nebulii  ;  Uerxcliel 
2041.  Ki;;ht  asceii!>ion,  S  hours  40  miu- 
utes ;  declinutiou,  69°  6'  S. 


452  THE  STELLAR   UNIVERSE. 

be  fewest  in  the  horizon,  and  will  increase  in  number  as  we  ap- 
proach the  zenith,  or  as,  going  below  the  horizon,  we  approach 
the  nadir.  The  positions  of  the  nebnlai  and  clusters  in  Sir  John 
Herschel's  great  catalogue  have  been  studied  by  Mr.  Cleve- 
land Abbe  with  especial  reference  to  their  distance  from  the 
galactic  circle,  and  the  following  numbers  show  part  of  liis  re- 
sults. Imagine  a  belt  thirty  degrees  wide  extending  around 
the  heavens,  including  the  Milky  Way,  and  reaching  fifteen 
degrees  on  each  side  of  the  central  circle  of  the  Milky  Way. 
This  belt  will  include  nearly  one-fourth  the  surface  of  the  ce- 
lestial sphere,  and  if  the  stars  or  nebnlai  were  equally  distrib- 
uted, nearly  one-fourth  of  them  would  be  found  in  the  belt. 
Instead,  however,  of  one-fourth,  we  find  nine-tenths  of  the  star 
clusters,  but  only  one-tenth  of  the  nebulae. 

The  discovery  that  the  nebula?  are  probably  masses  of  glow- 
ing gas  is  of  capital  importance  as  tending  to  substantiate  the 
view  of  Sir  William  Ilerschel,  that  these  masses  are  the  crude 
material  out  of  which  suns  and  systems  are  forming.  This 
view  was  necessarily  an  almost  purely  speculative  one  on  the 
part  of  that  distinguished  astronomer ;  but  unless  we  suppose 
that  the  nebuke  are  objects  of  almost  miraculous  power,  there 
must  be  some  trutli  in  it.  A  nebulous  body,  in  order  to  shine 
by  its  own  light,  as  it  does,  must  be  hot,  and  must  be  losing 
heat  through  the  very  radiation  by  which  w^c  see  it.  As  it 
cools,  it  must  contract,  and  this  contraction  cannot  cease  un- 
til it  becomes  either  a  solid  body  or  a  system  of  such  bodies 
revolving  round  each  other.  We  shall  explain  this  more  fully 
in  treating  of  cosmical  physics  and  the  nebular  hypothesis. 

§  7.  Proper  Motions  of  the  Stars. 

To  the  unassisted  eye,  the  stars  seem  to  preserve  the  same 
relative  .positions  in  the  celestial  sphere  generation  after  gen- 
eration. If  Job,  Ilipparchus,  or  Ptolemy  should  again  look 
npon  the  heavens,  he  would,  to  all  appearance,  see  Aldebaran, 
Orion,  and  the  Pleiades  exactly  as  he  saw  them  thousands  of 
years  ago,  without  a  single  star  being  moved  from  its  place. 
But  the  refined  methods  of  modern  astronomy,  in  which  the 


PROPER  MOTIONS  OF  THE  STARS.  453 

telescope  is  brought  in  to  measure  spaces  absolutely  invisible 
to  the  eye,  have  shown  that  this  seeming  nnchangeability  is 
not  real,  and  that  the  stars  are  actnally  in  motion,  only  the 
rate  of  change  is  so  slow  that  the  eye  wonld  not,  in  most  cases, 
notice  it  for  thonsands  of  years.  In  ten  thousand  years  qnite 
a  nnmber  of  stars,  especially  the  brighter  ones,  wonld  be  seen 
to  have  moved,  while  it  wonld  take  a  hundred  thousand  years 
to  introduce  a  very  noticeable  change  in  the  aspect  of  the  con- 
stellations. 

As  a  general  rule,  the  brighter  stars  have  the  greatest 
proper  motions.  But  this  is  a  rule  to  which  tliere  are  many 
exceptions.  The  star  which,  so  far  as  known,  has  the  greatest 
proper  motion  of  all — namely,  Groombridge  1830 — is  of  the 
seventh  magnitude  only.  Next  in  the  order  of  proj^er  motion 
comes  the  pair  of  stars  61  Cygni,  each  of  which  is  of  the  sixth 
magnitude.  Next  are  four  or  five  others  of  the  fourth  and 
fifth  magnitudes.  The  annual  motions  of  these  stars  are  as 
follows : 


Groombridge  1830 7".0 

61  Cygni n'.'i 

Lalande  21185 4".  7 

c  Indi 4  ".r> 


Lalande  21258 4". 4 

oMiiidani 4".l 

\k  Ca.ssiopeiiU 3".  8 

a  Centauri 3". 7 


The  fii'st  of  these  stars,  though  it  has  the  greatest  proper 
motion  of  all,  would  require  185,000  years  to  perform  the 
circuit  of  the  heavens,  while  fi  CassiopeiiB  wonld  require  near- 
ly 340,000  years  to  perform  the  same  circuit.  Slow  as  these 
motions  are,  they  are  very  large  compared  with,  those  of  most 
of  the  stars  of  corresponding  magnitude.  As  a  general  rule, 
the  stars  of  the  fourth,  fifth,  and  sixth  magnitudes  move  only 
a  few  seconds  in  a  hundred  years,  and  woiild  therefore  re- 
quire many  millions  of  years  to  perform  the  circuit  of  the 
heavens. 

So  far  as  they  have  yet  been  observed,  and,  indeed,  so  far 
as  they  can  be  observed  for  many  centuries  to  come,  these 
motions  take  place  in  perfectly  straight  lines.  If  each  star  is 
moving  in  some  orbit,  the  orbit  is  so  immense  that  no  curva- 
ture can  be  perceived  in  the  short  arc  which  has  been  de- 


454  THE  STELLAR   UNIVERSE. 

scribed  since  accurate  determinations  of  tlie  positions  of  the 
stars  be<):an  to  be  made.  So  far  as  mere  observation  can  in- 
foi-m  us,  ihere  is  no  reason  to  suppose  that  the  stars  are  sever- 
ally moving  in  definite  orbits  of  any  kind.  It  is  true  that 
Mildler  attempted  to  show,  from  an  examination  of  the  proper 
motions  of  the  stars,  that  the  whole  stellar  universe  Avas  revolv- 
ing around  the  star  Alcyone,  of  the  Pleiades,  as  a  centre — a 
theory  the  grandeur  of  which  led  to  its  wide  diffusion  in  popu- 
lar writings.  But  not  the  slightest  weight  has  ever  been  given 
it  by  astronomers,  who  have  always  seen  it  to  be  an  entirely 
baseless  speculation.  If  the  stars  were  moving  in  any  regular 
circular  orbits  whatever  having  a  connnon  centre,  we  could 
trace  some  regularity  among  their  proper  motions.  But  no 
such  regularity  can  be  seen.  The  stars  in  all  parts  of  the 
heavens  move  in  all  directions,  with  all  sorts  of  velocities.  It 
is  true  that,  by  averaging  the  proper  motions,  as  it  were,  we 
can  trace  a  certain  law  in  them  ;  but  this  law  indicates,  not  a 
])articular  kind  of  orbit,  but  only  an  apparent  proper  motion, 
common  to  all  the  stars,  which  is  probably  due  to  a  real  mo- 
tion of  our  sun  and  solar  system. 

The  Solar  Mutton. — As  our  sun  is  merely  one  of  the  stars, 
and  rather  a  small  star  too,  it  may  have  a  proper  motion  as 
well  as  the  other  stars.  Moreover,  when  we  speak  of  the 
proper  motion  of  a  star,  we  mean,  not  its  absolute  motion,  but 
only  its  motion  relative  to  our  system.  As  the  sun  moves,  he 
carries  the  earth  and  all  the  planets  along  with  him ;  and  if 
we  observe  a  star  at  perfect  rest  while  we  ourselves  are  thus 
moving,  the  star  will  appear  to  move  in  the  opposite  direc- 
tion, as  we  have  already  shown  in  explaining  the  Copernican 
system.  Hence,  from  an  observation  of  the  motion  of  a  sin- 
gle star,  it  is  impossible  to  decide  how  much  of  this  apparent 
motion  is  due  to  the  motion  of  our  system,  and  how  much  to 
the  real  motion  of  the  star.  If,  however,  we  should  observe  a 
great  number  of  stars  on  all  sides  of  u  '>,  and  find  them  all  ap- 
parently moving  in  the  same  direction,  it  would  be  natural  to 
conclude  that  it  was  really  our  system  which  was  moving,  and 
not  the  stars.     Now,  when  Herschel  averaged  the  proper  mo- 


r  no  PER   MOTIONS  OF  THE  STARS. 


455 


tioiis  of  tlio  stars  in  different  regions  of  the  heavens,  he  fonntl 
that  this  was  actually  the  case.  In  general,  the  stars  moved 
from  the  direction  of  the  constellation  Hercules,  and  towards 
the  opposite  point  of  the  celestial  sphere,  near  the  constella- 
tion Argns.  This  would  show  that,  relatively  to  the  general 
mass  of  the  stars,  our  sun  was  moving  in  the  direction  of  the 
constellation  Hercules.  Ilerschel's  data  for  this  conclusion 
were,  necessarily,  rather  slender.  The  subject  was  afterwards 
very  carefully  investigated  by  Argelander,  and  then  by  a  num- 
ber of  other  astronomers,  whose  results  for  the  point  of  the 
heavens  towards  which  the  sun  is  moviniii:  are  as  follows : 


Hi^ht  AHceiision. 

Declhi'ttion. 

Argelimcler 

( ).  Stni ve 

257"  4!)' 
2(51"  22' 
252"  24' 

2(;()^    1' 

2(11  '  ;58' 
2(;2"  29' 

28"  50'  N. 
;^7"  8(i'  N. 
14"  2(i'  N. 
M'  23 '  N. 
;i9"  54'  N. 
28"  58'  N. 

Liiiuliilil 

( lallowiiv 

M'adler 

Airy  and  Dmikin 

It  will  be  seen  that  while  there  is  a  pretty  wide  range  among 
the  authorities  as  to  the  exact  ])oint,  and,  therefore,  some  un- 
certainty as  to  where  we  should  locate  it,  yet,  if  we  lay  the 
different  points  down  on  a  star-map,  we  shall  find  that  they 
all  fall  in  the  constellation  Hercules,  which  was  originally  as- 
signed by  Ilerschel  as  that  towards  which  we  were  moving. 

As  to  the  amount  of  the  motion,  Struve  found  that  if  the 
sun  were  viewed  from  the  distance  of  an  average  star  of  the 
first  magnitude  placed  in  a  direction  from  us  at  right  angles 
to  that  of  the  solar  motion,  it  would  appear  to  move  at  the 
rate  of  33".9  per  century.  Dunkin  found  the  same  motion  to 
be  33".5  or  41".0,  according  to  the  use  he  made  of  stars  hav- 
ing large  proper  motions. 

Motion  of  Groiqys  of  Stars. — There  are  in  the  heavens  sev- 
eral cases  of  widely  extended  groups  of  stars,  having  a  com- 
mon proper  motion  entirely  different  from  that  of  the  stars 
around  and  among  them.  Such  groups  must  form  connected 
systems,  in  the  motion  of  which  all  the  stars  are  carried  along 
together  without  any  great  change  in  their  positions  relative 


456  THE  STELLAR   UNIVERSE. 

to  each  otlier.  The  most  remarkable  case  of  this  kind  oc- 
curs in  tlie  constelhitiou  Taurus.  A  lar^e  n)ajority  of  the 
brighter  ^tars  in  the  region  between  Akleburan  and  the  Plei- 
ades have  a  common  proper  motion  of  about  ten  seconds  per 
century  towards  the  east.  How  many  stars  are  included  in 
this  group  no  one  knows,  as  the  motions  of  the  brighter  ones 
only  have  been  accurately  investigated.  Mr.  II.  A.  Proctor 
has  shown  that  five  out  of  the  seven  stars  which  form  the 
Dipper,  or  Great  Pear,  are  similarly  connected,  lie  proposes 
for  this  comnumity  of  proper  motions  in  certain  regions  the 
name  of  /Slar-dri/L  Pesides  those  we  have  mentioned,  there 
are  cases  of  close  groups  of  stars,  like  the  Pleiades,  and  of 
pai'~  of  widely  separated  stars,  in  which  star- drift  has  been 
noticed. 

Motion  hi  the  Line  of  Sight. — Until  (piite  recently,  the  only 
way  in  which  the  proper  motion  of  a  star  could  be  detected 
was  by  observing  its  change  of  direction,  or  the  change  of  the 
point  in  which  it  is  seen  on  the  celestial  sphere.  It  is,  how- 
ever, impossible  in  this  way  to  decide  whether  the  star  is  or  is 
not  changing  its  distance  from  our  system.  If  it  be  mo^■ing 
directly  towards  us,  or  directly  away  from  us,  we  could  not 
see  any  motion  at  all.  The  complete  motion  of  the  stars  can- 
not, therefore,  be  determined  by  mere  telescopic  observaiions. 
Put  there  is  an  higenious  method,  founded  on  the  undulatory 
theory  of  light,  by  which  this  motion  niay  be  detected  with 
more  or  less  probability  by  means  of  the  spectroscope,  and 
which  was  first  successfully  applied  by  Mr.  Iluggins,  of  Eng- 
land. According  to  the  usual  theory  of  light,  the  luminosity 
of  a  heated  body  is  a  result  of  the  vibrations  communicated 
by  it  to  the  ethereal  medium  which  fills  all  space ;  and  if  the 
body  be  gaseous,  it  is  supposed  that  a  molecule  of  the  gas  vi- 
brates at  a  certain  definite  rate,  and  thus  communicates  only 
certain  definite  vibrations  to  the  ether.  The  rate  of  vibration 
is  determined  by  the  position  of  the  bright  line  in  the  spec- 
trum of  the  gas.  Now,  if  the  vibrating  body  be  moving 
through  the  ether,  the  light-waves  which  it  throws  behind  it 
will  be  longer,  and  those  which  it  throws  in  front  of  it  will  be 


PROPER  MOTIONS  OF  THE  STARS.  457 

shorter,  than  if  the  body  were  at  rest.  The  result  will  be,  that 
in  tlie  former  ease  the  spectral  lines  will  be  less  refrangible, 
or  nearer  the  red  end  of  the  si)e'*truni,  and  in  the  latter  case 
nearer  the  blue  end.  If  the  line  is  liot  a  bright  one  which  the 
gas  emits,  bnt  the  corresponding  dark  one  which  it  has  ab- 
sorbed from  the  light  of  a  star  passing  through  it,  the  result 
will  be  the  same.  If  sucii  a  Iiiown  line  is  found  slightly 
nearer  the  blue  end  of  the  spectrum  than  it  should  be,  it  is 
concluded  that  the  star  from  which  it  emanates  is  aj)proacli- 
ing  us,  while  in  the  contrary  case  it  is  receding  from  ns. 

The  question  may  be  asked.  How  can  we  identify  a  line  as 
proceeding  from  a  gas,  unless  it  is  exactly  in  the  position  of 
the  line  due  to  that  gas  ?  How  do  we  know  but  that  it  may 
be  due  to  some  other  gas  which  emits  light  of  slightly  differ- 
ent refrangibility  ?  The  reply  to  this  must  be,  that  absolute 
certainty  on  this  point  is  not  attainable  ;  but  that,  from  the 
examination  of  a  number  of  stars,  the  probabilities  seem  large- 
ly in  favor  of  the  opinion  that  the  disj)laced  lines  are  really 
due  to  the  gases  near  whose  lines  they  fall.  If  the  lines  were 
always  displaced  in  one  direction,  whatever  star  was  exam- 
ined, the  conclusion  in  question  could  not  be  drawn,  because 
it  micrht  be  that  this  line  was  due  to  some  other  unknown  sub- 
stance.  But  as  a  matter  of  fact,  when  different  stars  are  ex- 
amined, it  is  found  that  the  lines  in  question  are  sometimes 
on  one  side  of  their  normal  position  and  sometimes  on  the 
other.  This  makes  it  probable  that  they  really  all  belong  to 
one  substance,  but  are  displaced  by  some  ciuse,  and  the  motion 
of  the  star  is  a  cause  the  existence  of  which  is  certain,  and  the 
sufficiency  of  which  is  probable. 

Mr.  Iluggins's  system  of  measurement  has  been  introduced 
by  Professor  Airy  into  the  Royal  Observatory,  Greenwioli, 
where  very  careful  measures  have  been  made  during  the  past 
two  years  by  Mr.  Christie  and  Mr.  Maunder.  To  show  how 
well  the  fact  of  the  motion  is  made  out,  we  give  in  the  tables 
on  the  following  page  the  results  obtained  by  Mr.  lluggins 
and  by  the  Greenwich  observers  for  those  stars  in  which  the 
motion  is  the  largest : 


458 


THE  STELLAR   UNIVERSE. 


STARS    UKCEDING   FROM    US. 


n.v  Mr.  HuRitiiis. 

Ily  (Jrceriwii-h. 

Siriiis 

20  miles  per  sec. 
22     "           " 
15     "           " 
25     "           " 
15     " 

25  miles  per  sec. 

7G     " 

receding. 

25  miles  per  sec. 

80     "          " 

a  Orionis 

/3  Orionis 

rt  Geminonim 

a  Lieonis 

STARS    APPKOACIIING    US. 


By  Mr.  Ilii^giiiH. 

By  (JreenwH-h. 

Arcturus 

55  miles  per  sec. 
.50     "           " 

;?!)    "        " 

4!)     "           " 
4(!     "           " 

41  miles  per  sec. 
;5(!     " 
41     " 

npj)roa('liing. 
ap))roaciiing. 

a  Lvrtc 

a  C'vgni 

/3  Geminonim 

a  Ursat!  Miiioris 

Tlierc  are  several  collateral  circumstances  wliicli  tend  to 
confirm  these  results.  One  is  tliat  the  general  amount  of  mo- 
tion indicated  is,  in  a  rough  M'ay,  about  what  we  should  expect 
the  stars  to  have,  from  their  observed  proper  motions,  com- 
bined with  their  probable  parallaxes.  Another  is  that  those 
stars  in  the  neighborhood  of  Hercules  are  mostly  found  to  be 
approaching  the  earth,  and  those  which  lie  in  the  opposite  di- 
rection to  be  receding  from  it,  which  is  exactly  the  effect  which 
would  result  from  the  solar  motion  just  described.  Again,  the 
five  stars  in  the  Dipper  which  we  have  described  as  having  a 
common  proper  motion  are  also  found  to  have  a  common  mo- 
tion in  the  line  of  sight.  The  results  of  this  wonderful  and 
refined  method  of  determining  stellar  motion,  therefore,  seem 
worthy  of  being  received  with  some  confidence  so  far  as  the 
general  direction  of  the  motion  is  concerned.  But  the  dis- 
placement of  the  spectral  lines  is  so  slight,  and  its  measure- 
ment a  matter  of  such  difficulty  and  delicacy,  that  we  are  far 
from  being  sure  of  the  exact  numbers  of  miles  per  second 
given  by  the  observers.  The  discordances  between  the  results 
of  Greenwich  and  those  of  Mr.  Iluggins  show  that  numerical 
certainty  is  not  yet  attained. 

A  necessary  result  of  these  motions  will  be  that  those  stai-s 
which  are  receding  from  us  will,  in  the  course  of  ages,  appear 
less  brilliant,  owing  to  their  greater  distance,  while  those  which 


rnOl'ER  MOTIONS  OF  THE  STABS.  459 

are  approaching  ns  will,  as  they  come  nearer,  appear  brighter, 
always  supposing  tliat  their  intrinsic  brightness  does  not  vary. 
13iit  so  innnense  is  the  distance  of  the  stars,  that  many  thou- 
sands of  years  will  be  required  to  produce  any  appreciable 
change  in  their  brightness  from  this  cause.  For  instance, 
from  the  best  determinations  which  have  been  made,  the  dis- 
tance of  Sirius  from  our  system  is  more  than  a  million  radii 
of  the  earth's  orbit.  With  a  velocity  of  twenty  miles  per  sec- 
ond, it  would  require  more  than  one  hundred  and  fifty  thou- 
sand years  to  pass  over  this  distance. 

It  will,  of  course,  be  understood  that  the  velocities  found  by 
the  spectroscopic  method  are  not  the  total  velocities  with 
which  the  stars  are  moving,  but  only  the  rate  at  which  they 
are  approaching  to  or  receding  from  the  earth,  or,  to  speak 
mathematically,  the  component  of  the  velocity  in  the  direc- 
tion of  the  line  of  sight.  To  find  the  total  velocity,  this  com- 
ponent must  be  combined  W'itli  the  telescopic  velocity  found 
from  the  obser\'ed  proper  motion  of  the  star,  which  is  the  ve- 
locity at  right  angles  to  the  line  of  sight.  Xone  of  the  stars 
are  moving  exactly  towards  our  system,  and  it  is  not  likely 
that  any  will  ever  pass  very  near  it.  In  the  preceding  list, 
the  star  a  Cygni  is  the  one  wliicli  is  coming  most  directly 
towards  vs.  Its  telescopic  proper  motion  is  so  slight  that, 
though  we  suppose  its  distance  to  be  two  million  radii  of  the 
earth's  orbit,  yet  its  velocity  at  right  angles  to  the  line  of  sight 
will  hardly  amount  to  one-third  of  a  mile  per  second.  If  the 
spectroscopic  determination  is  correct,  then,  after  an  interval 
which  will  probably  fall  between  one  hundred  thousand  and 
three  hundred  thousand  years,  a  Cygni  will  pass  by  our  sys- 
tem at  something  like  a  hundredth  of  its  present  distance, 
and  will,  for  several  thousand  years,  be  many  times  nearer  and 
brighter  than  any  star  is  now. 


460  TUE  STELLAR   UNIVERSE. 


CHAPTER  11. 

THE    STRUCTURE   01    THE    UNIVERSE. 

Having  in  the  pi'eceding  chapter  debcribed  those  features 
of  the  universe  whicli  the  telescope  exhibits  to  us,  we  have 
now,  in  pui*suance  of  our  plan,  to  inquire  what  light  telescopic 
discoveries  can  throw  upon  the  structure  of  the  universe  as  a 
whole.  Here  we  necessarily  ti-ead  upon  ground  less  sure  than 
that  which  has  hitherto  supported  tis,  because  w  are  on  the 
very  boundaries  of  human  knowledge.  Many  of  our  conclu- 
sions must  be  more  or  less  hypothetical,  and  liable  to  be  modi- 
fied or  disproved  by  subsequent  discoveries.  We  shall  en- 
deavor to  avoid  all  mere  guesses,  and  to  state  no  conclusion 
which  has  not  some  apparent  foundation  in  observation  or 
analogy.  The  human  mind  cannot  be  kept  from  speculating 
upon  and  wondering  about  the  order  of  creation  in  its  widest 
extent,  and  science  will  be  doing  it  a  service  in  throwing  ev- 
ery possible  light  on  its  path,  and  preventing  it  from  reaching 
any  conclusion  inconsistent  with  observed  facts. 

The  first  question  which  we  reach  in  regular  order  is,  How 
are  the  forty  or  fifty  millions  of  stars  visible  in  the  most  pow- 
erful telescopes  arranged  in  space  ?  We  know,  from  direct 
observation,  how  they  are  arranged  with  respect  to  direction 
from  our  system ;  and  we  have  seen  that  the  vast  majority  of 
small  stars  visible  in  great  telescopes  are  found  in  a  belt  span- 
ning the  heavens,  and  known  as  the  Milky  Way.  But  this 
gives  us  no  complete  information  respecting  their  absolute  po- 
sition :  to  determine  this,  we  must  know  the  distance  as  well 
as  the  direction  of  each  star.  But  beyond  the  score  or  so  of 
stars  which  have  a  measurable  parallax,  there  is  no  know^n 
way  of  measuring  the  stellar  distances;  so  that  all  we  can  do 


VIE  TVS  OF  MODERN  ASTRONOMERS.  461 

is  to  make  more  or  less  probable  conjectures,  founded  on  the 
apparent  magnitude  of  the  individual  stars  and  the  probable 
laws  of  their  arrangement.  If  the  stars  were  all  of  the  same 
intrinsic  brightness,  we  could  n.ake  a  very  good  estimate  of 
their  distance  from  their  apparent  magnitude ;  but  we  know 
that  such  is  n^t  the  case.  Still,  in  all  reasonable  probability, 
the  diversity  of  absolute  magnitude  is  far  less  tJian  that  of  the 
apparent  magnitude;  so  that  a  judgment  founded  on  the  lat- 
ter is  much  better  than  none  at  all.  It  was  on  such  consider- 
ations as  these  that  the  conjectures  of  the  first  observers  with 
the  telescope  were  founded. 

§  1.  Views  of  Astronomers  before  Ilerschel 

Before  the  invention  of  the  telescope,  any  well-founded 
opinion  respecting  the  structure  of  the  starry  system  was  out 
of  the  question.  We  have  seen  how  strong  a  hold  the  idea  of 
a  spherical  universe  had  on  the  minds  of  men,  so  that  even 
Copernicus  was  fully  possessed  with  it,  and  probably  believed 
the  sun  to  be,  in  some  way,  the  centre  of  this  sphere.  Before 
any  step  could  be  taken  towards  forming  a  true  conception  of 
the  universe,  this  idea  had  to  be  banished  from  the  mind,  and 
the  sun  had  to  be  recognized  as  simply  one  of  iimumerable 
stars  which  made  up  the  universe.  The  possibility  that  such 
might  have  been  the  case  seems  to  have  first  suggested  itself 
to  Kepler,  though  he  was  deterred  from  completely  accepting 
the  idea  by  an  incorrect  estimate  of  the  relative  brilliancy  of 
the  stars.  lie  reasoned  that  if  the  sun  were  one  of  a  vast 
number  of  fixed  stars  of  equal  brilliancy  scattered  uniformly 
throughout  space,  there  could  not  be  more  than  twelve  which 
wei'e  at  the  shortest  distance  from  us.  We  should  then  have 
another  set  at  double  the  distance,  another  at  triple  the  dis- 
tance, and  so  on  ;  and  since  the  more  distant  they  are,  the 
fainter  they  would  appear,  we  should  speedily  reach  a  limit 
beyond  which  no  stars  could  be  seen.  In  fact,  however,  we 
often  see  numerous  stars  of  the  same  magnitude  crowded 
closely  together,  as  in  the  belt  of  Orion,  while  the  total  num- 
ber of  visible  stars  is  reckoiicd  by  thousands.     He  therefore 


462  THE  STELLAR   UNIVERSE. 

concludes  that  the  distances  of  the  individual  stars  from  each 
other  are  much  less  than  their  distances  from  our  sun,  the  lat- 
ter being  situated  near  the  centre  of  a  comparatively  vacant 
region. 

Had  Kepler  known  that  it  would  require  the  light  of  a  hun- 
dred stars  of  the  sixth  magnitude  to  make  that  of  one  of  the 
first  magnitude,  he  would  not  have  reached  this  conclusion. 
A  simple  calculation  would  have  sho^n  him  that,  with  twelve 
stars  at  distance  unity,  there  would  have  been  four  times  that 
number  at  the  double  distance,  nine  times  at  the  treble  dis- 
tance, and  so  on,  until,  within  the  tenth  sphere,  there  would 
have  been  more  than  four  thousand  stars.  The  twelve  liun- 
dred  stars  on  the  surface  of  the  tenth  sphere  would  have 
been,  by  calculation,  of  the  sixth  magnitude,  a  number  near 
enough  to  that  given  by  actual  count  to  show  him  that  tlie 
hypothesis  of  a  uniform  distribution  was  quite  accordant  with 
observations.  It  is  true  that,  where  many  bright  stars  M'ere 
found  crowded  together,  as  in  Orion,  their  distance  from  each 
otlier  is  probably  less  than  that  from  our  sun.  But  this  ag- 
glomeration, being  quite  exceptional,  would  not  indicate  a  gen- 
eral crowding  together  of  all  the  stars,  as  Kepler  seemed  to 
suppose.  In  justice  to  Kepler  it  must  be  said  that  he  put 
forth  this  view,  not  as  a  well-founded  theory,  but  only  as  a 
surmise,  concerning  a  question  in  which  certainty  was  not 
attainable. 

Ideas  of  Kant. — Those  who  know  of  Kant  only  as  a  specula- 
tive philosopher  may  be  surprised  to  learn  that,  although  he 
was  not  a  working  astronomer,  he  was  the  author  of  a  theory 
of  the  stellar  system  which,  with  some  modifications,  has  been 
very  generally  held  until  the  present  time.  Seeing  the  Gal- 
axy encircle  the  heavens,  and  knowing  it  to  be  produced  by 
the  light  of  innumerable  stars  too  distant  to  be  individually 
visible,  he  concluded  that  the  stellar  system  extended  much 
farther  in  the  direction  of  tlie  Galaxy  than  it  did  elsewhere. 
In  other  words,  he  conceived  the  stars  to  be  arranged  in  a 
comparatively  thin,  fiat  layer,  or  stratum,  our  sun  being  some- 
where near  the  centre.     When  we  look  edgewise  along  this 


VIE  TVS  OF  MODERN  ASTRONOMERS.  463 

Btratnm,  we  see  an  immense  number  of  stars,  but  in  the  per- 
pendicular direction  comparatively  few  are  visible.* 

This  thin  stratum  suggested  to  Kant  the  idea  of  a  certain 
resemblance  to  the  solar  system.  Owing  to  the  small  inclina- 
tions of  the  planetary  orbits,  the  bodies  which  compose  this 
system  are  spread  out  in  a  tliin  layer,  as  it  were ;  and  we  have 
only  to  add  a  great  multitude  of  planets  moving  around  tb« 
sun  in  orbits  of  varied  inclinations  to  have  a  representation  in 
miniature  of  the  stellar  system  as  Kant  imagined  it  to  exist. 
Had  the  zone  of  small  planets  between  Ma>'s  and  Jupiter  then 
been  known,  it  would  have  afforded  a  striking  confirmation  of 
Kant's  view  by  showing  a  yet  greater  resemblance  of  the  plan- 
etary system  to  his  supposed  stellar  system.  Were  the  mnn- 
ber  of  these  small  planets  sufficiently  increased,  we  should  see 
them  as  a  sort  of  Galaxy  around  the  zodiac,  a  second  Milky 
Way,  belonging  to  our  system,  and  resolvable  witli  the  tele- 
scope into  small  planets,  just  as  the  Galaxy  is  resolved  into 
small  stars.  The  conclusion  that  two  systems  which  were  so 
similar  in  appearance  M'ere  really  alike  in  structure  would 
have  seemed  very  well  founded  in  analogy. 

As  the  planets  are  kept  at  their  proper  distances,  and  pre- 
vented from  falling  into  each  other  or  into  the  sun  by  the 
centrifugal  force  generated  by  their  revolutions  in  their  or- 
bits, so  Kant  supposed  the  stars  to  be  kept  apait  by  a  revolu- 
tion around  some  common  centre.  The  proper  motions  of 
the  stars  were  then  almost  unknown,  and  the  objection  was 
anticipated  that  the  stars  were  found  to  occupy  the  same  po- 
sition in  the  heavens  from  generation  to  generation,  and  there- 
fore could  not  be  in  motion  around  a  centre.  To  this  Kant's 
reply  was  that  the  time  of  revolution  was  so  long,  and  the 
motion  so  slow,  that  it  was  not  perceptible  with  the  imper- 
fect means  of  observation  then  available.  Future  ffcnora- 
tions  would,  he  doubted  not,  by  comparing  their  observations 


*  The  onp;innl  idea  of  this  theory  is  attributed  by  Kant  to  Wright,  of  Durliam, 
Enghuid,  a  writer  whose  woiks  are  entirely  nnknown  in  tliis  country,  and  whose 
authorship  of  the  tlieory  has  been  very  generally  forgotten. 


464  THE  STELLAR   UNIVERSE. 

with  tliosc  of  their  predecessors,  find  that  there  actually  was  a 
motion  anionfr  the  stars. 

This  conjecture  of  Kant,  that  the  stars  would  be  found  to 
have  a  proper  motion,  has,  as  we  have  seen,  been  amply  con- 
firmed ;  but  tlie  motion  ia  not  of  the  kind  which  his  theory 
would  require.  On  this  theory,  all  the  stars  ought  to  move  in 
directions  nearly  parallel  to  that  of  the  Milky  Way,  just  as  in 
the  planetary  system  we  find  them  all  moving  in  directions 
nearly  parallel  to  the  ecliptic.  But  the  proper  motions  actually 
observed  have  no  common  direction,  and  follow  no  law  what- 
ever, except  that,  on  the  average,  there  is  a  preponderance  of 
motions  from  the  constellation  Hercules,  which  is  attributed 
to  an  actual  motion  of  our  sun  in  that  direction.  Making  al- 
lowance for  this  preponderance,  M'e  find  the  stars  to  be  appar- 
ently moving  at  random  in  every  direction ;  and  therefore 
they  cannot  be  moving  in  any  regularly  arranged  orbits,  as 
Kant  su])posed.  A  defender  of  Kant's  system  might  indeed 
maintain  that,  as  it  is  only  in  a  few  of  the  stars  nearest  us 
that  any  proper  motion  has  been  detected,  the  great  cloud  of 
stars  which  make  up  the  Milk}'  Way  might  really  be  moving 
along  in  regular  order,  a  view  the  possibility  of  which  we  shall 
be  better  prepared  to  consider  hereafter. 

The  Kantian  theory  supposes  the  system  which  we  have 
just  been  describing  to  be  formed  of  the  immense  stratum  of 
stars  which  make  up  the  Galaxy  and  stud  our  heavens,  and 
to  include  all  the  stars  separately  visible  with  our  telescopes. 
But  he  did  not  suppose  this  system,  immense  though  it  is,  to 
constitute  the  whole  material  universe.  In  the  nebuljE  he 
saw  other  similar  systems  at  distances  so  immense  that  the 
combined  light  of  their  millions  of  suns  only  appeared  as  a 
faint  cloud  in  the  most  powerful  telescopes.  This  idea  that 
the  nebultc  were  other  galaxies  was  more  or  less  in  vogue 
am.ong  popular  writers  until  a  quite  recent  period,  when  it 
was  refuted  by  the  spectroscope,  which  shows  that  these  ob- 
jects are  for  the  most  part  masses  of  glowing  gas.  It  has, 
however,  not  received  support  among  astronomers  since  the 
time  of  Sir  William  Ilerschel. 


HERSCHEL  AND  HIS  SUCCESSOBS.     465 

System  of  Lambert. — A  few  years  after  the  appearance  of 
Kant's  work,  a  similar  but  more  elaborate  sj'stem  was  sketched 
out  by  Lambert.  lie  supposed  the  universe  to  be  arranged  in 
systems  of  different  orders.  The  smallest  systems  which  we 
know  are  those  made  up  of  a  planet,  with  its  satellites  circu- 
lating around  it  as  a  centre.  The  next  system  in  order  of 
magnitude  is  a  solar  system,  in  which  a  number  of  smaller 
systems  are  each  carried  round  the  sim.  Each  individual  star 
which  we  see  is  a  sun,  and  has  its  retinue  of  planets  revolving 
around  it,  so  that  there  are  as  many  solar  systems  as  stars. 
These  systems  are  not,  however,  scattered  at  random,  but  are 
divided  up  into  greater  systems  which  appear  in  our  telescopes 
as  clusters  of  stars.  An  immense  number  of  these  clusters 
make  up  our  Galaxy,  and  form  the  visible  universe  as  seen  in 
our  telescopes.  There  may  be  yet  greater  systems,  each  made 
up  of  galaxies,  and  so  on  indefinitely,  only  their  distance  is  so 
immense  as  to  elude  our  observation. 

Each  of  the  smaller  systems  visible  to  us  has  its  central  body, 
the  mass  of  which  is  much  greater  than  that  of  those  which 
revolve  around  it.  This  feature  Lambert  supposed  to  extend 
to  other  systems.  As  the  planets  are  larger  than  their  satel- 
lites, and  the  sun  larger  than  its  planets,  so  he  supposed  each 
stellar  cluster  to  have  a  great  central  body  around  which  each 
solar  system  revolved.  As  these  central  bodies  are  invisible  to 
us,  he  supposed  them  to  be  opaque  and  dark.  All  the  systems, 
from  the  smallest  to  the  greatest,  were  supposed  to  be  bound 
together  by  the  one  universal  law  of  gravitation. 

As  not  the  slightest  evidence  favoring  the  existence  of  these 
opaque  centres  has  ever  been  found,  we  are  bound  to  say  that 
this  sublime  idea  of  Lambert's  has  no  scientific  foundation. 
Astronomers  have  handed  it  over  without  reservation  to  the 
lecturers  and  essayists. 

§  2.  Researches  of  Herschel  and  his  Successors. 

Ilerschel  was  the  first  who  investigated  the  structure  of 
the  stellar  system  by  a  long-continued  series  of  observations, 
executed  with  a  definite  end  in  view.    His  plan  was  that  of 

31 


4G6  THE  STELLAR  UNIVERSE. 

"  star  -  gauging,"  which  meant,  in  the  first  place,  the  simple 
enumeration  of  all  the  stars  visible  with  a  powerful  tele- 
scope in  a  given  portion  of  the  heaNens.  He  emplo^'ed  a 
telescope  of  twenty  inches  aperture,  magnifying  one  hundred 
and  sixty  times,  the  field  of  view  be  ig  a  quarter  of  a  degree 
in  diameter.  This  diameter  was  a^ujut  lut.f  tliat  of  the  full 
moon,  so  that  each  count  or  gauge  included  all  the  stars  visi- 
ble in  a  space  having  one-fourth  the  apparent  surface  of  the 
lunar  disk.  From  the  number  of  stars  in  any  one  field  of 
view,  he  concluded  to  w^hat  relative  distance  his  sight  ex- 
tended, supposing  a  uniform  distribution  of  the  stars  through- 
out all  the  space  included  in  the  cone  of  sight  of  the  telescope. 
When  an  observer  looks  into  a  telescope  pointed  at  the  heav- 
ens, his  field  of  vision  includes  a  space  which  constantly 
widens  out  on  all  sides  as  the  distance  becomes  greater ;  and 
the  reader  acquainted  with  geox^etry  Avill  see  that  this  space 
forms  a  cone  having  its  point  in  the  focus  of  the  telescope,  and 
its  circular  base  at  the  extreme  distance  to  which  the  telescope 
reaches.  The  solid  contents  of  this  cone  will  be  proportional 
to  the  cube  of  the  distance  to  which  it  extends ;  for  instance, 
if  the  telescope  penetrates  twice  as  far,  the  cone  of  sight  will 
be  not  only  twice  as  long,  but  the  base  will  be  twice  as  wide 
in  each  direction,  so  that  the  cone  will  have  altogether  eight 
times  the  contents,  and  will,  on  Ilerschel's  hypotliesis,  contain 
eight  times  as  many  stars.  So,  when  Ilerschel  found  the  stars 
eight  times  as  numerous  in  one  region  as  in  another,  he  con- 
cluded that  the  stellar  svstem  extended  twice  as  far  in  the 
direction  of  the  first  region. 

To  count  all  the  stars  visible  with  his  telescope,  Ilerschel 
found  to  be  out  of  the  question.  He  would  have  had  to  point 
his  instrument  several  hundred  thousand  times,  and  count  all 
the  visible  :  Lars  at  each  ;3ointing.  He  therefore  extended  his 
survey  only  over  a  wide  belt  extending  more  than  half-way 
round  the  celestial  sphere,  and  cutting  the  Galaxy  at  right 
angles.  In  this  belt  he  counted  the  stars  in  3400  telcECopic 
fields.  Comparing  the  average  number  of  stars  in  different 
regions  with  the  position  of  the  region  relative  to  the  Galaxy, 


RESEARCHES   OF  UERSCHEL  AND  HIS  SUCCESSORS.     467 

he  found  that  the  stars  were  thinnest  at  the  point  most  distant 
from  the  Galaxy,  and  that  they  constantly  increased  in  num- 
ber as  the  Galaxy  was  approached.  The  following  table  will 
give  an  idea  of  the  rate  of  increase.  It  shows  the  average 
number  of  stars  in  the  field  of  view  of  the  telescope  for  each 
of  six  zones  of  distance  from  the  Galaxy, 

First  zone 90°  to  75°  from  Galaxy 4  stars  per  field. 

Secondzone 75°  "  G()°     "         "     ' 5       " 

Third  zone C0°  "  45°     "         "       8       "         " 

Fourth  Kone 45°  "  .30°     "         "       14       "         " 

Fifth  zone 80°  "  15°     "        "       24       "         " 

Sixth  zone 15°"    0°     "        "       53      " 

A  similar  enumeration  was  made  l)y  Sir  John  Ilerschel  for  the 
corresponding  region  on  the  other,  or  southern,  side  of  the  Gal- 
axy, lie  used  the  same  telescope,  and  the  same  magnifying 
power.     Ilis  results  were : 

First  zone G  stars  per  field.  !  Fourth  zone 13  stars  per  field. 


Secondzone 7 

Third  zone 1) 


Fifth  zone 2i'> 

Sixth  zone 5U 


The  reader  will,  perhaps,  more  readily  grasp  the  significa- 
tion of  these  numbers  by  the  mode  of  representation  which 
was  suggested  in  describing  the  distribution  of  the  nebulne. 
Let  him  imagine  himself  standing  under  a  clear  sky  at  the 
time  when  the  Milky  Way  encircles  the  horizon.  Then,  tho 
first  zone,  as  we  have  defined  it,  will  be  around  the  zenith,  ex- 
tending one -sixth  of  the  way  to  the  horizon  on  every  side; 
the  second  zone  will  be  next  below  and  around  this  circular 
space,  extending  one-third  of  the  way  to  the  horizon ;  and  so 
each  one  will  follow  in  regular  order  until  we  reach  the  sixth, 
or  galactic,  zone,  which  will  encircle  the  horizon  to  a  height 
of  15°  on  every  side.  The  numbers  we  have  given  show  that 
in  the  position  of  the  observer  which  we  have  supposed  the 
stars  would  be  thinnest  around  the  zenith,  and  would  con- 
stantly increase  in  number  as  we  approached  the  horizon. 
The  observer  being  supposed  still  to  occupy  the  same  posi- 
tion, the  second  table  shows  the  distribution  of  the  stars  in  the 


4G8  THE  STELLAR   UNI  VERSE. 

opposite  or  invisible  hemisphere,  which  he  would  see  if  the 
earth  were  removed.  In  this  hemisphere  the  fii*st,  or  thinnest, 
zone  would  be  directly  opposite  the  thinnest  zone  in  the  ob- 
server's zenith  ;  that  is,  it  would  be  directly  under  his  feet. 
The  successive  zones  would  then  be  nearer  the  horizon,  the 
sixth  or  last  encircling  it,  and  extending  15°  below  it  on  every 
side. 

The  numbers  we  have  given  are  only  averages,  and  do  not 
give  an  adequate  idea  of  tlie  actual  inequalities  of  distribu- 
tion in  special  regions  of  the  heavens.  Sometimes  there  was 
not  a  solitary  star  in  the  field  of  the  telescope,  while  at  oth- 
ers there  were  many  hundreds.  In  the  circle  of  the  Galaxy 
itself,  the  stars  are  more  than  twice  as  thick  as  in  the  average 
of  the  first  zone,  which  includes  not  only  this  circle,  but  a 
space  of  15°  on  each  side  of  it. 

Adopting  the  hypothesis  of  a  uniform  distribution  of  the 
stars,  Ilerschel  concluded  from  his  first  researches  that  the 
stellar  system  was  of  the  general  form  supposed  by  Kant,  ex- 
tending out  on  all  sides  five  times  as  far  in  the  direction  of 
the  Galaxy  as  in  the  direction  perpendicular  to  it.  The  most 
important  modification  he  made  was  to  suppose  an  immense 
cleft  extending  edgewise  into  the  system  from  its  circumfer- 
ence about  half-way  to  the  centre.  This  cleft  corresponded  to 
the  division  in  the  Milky  Way  which  commences  in  the  sum- 
mer constellation  Cygnus  in  the  north,  and  passes  through 
Aquila,  the  Serpent,  and  Scorpius  far  into  the  southern  hemi- 
sphere. Estimating  the  distance  by  the  arrangement  and  ap- 
parent magnitude  of  the  stars,  he  was  led  to  estimate  the  mean 
thickness  of  the  stellar  stratum  from  top  to  bottom  as  155 
units,  and  the  diameter  as  850  units,  the  unit  being  the  aver- 
age distance  of  a  star  of  the  first  magnitude.  Supposing  this 
distance  to  be  that  which  light  would  travel  over  in  16  years 
— a  supposition  which  is  founded  on  the  received  estimate  of 
the  mean  parallax  corresponding  to  stars  of  that  magnitude — 
then  it  would  take  light  nearly  14,000  years  to  travel  across 
the  system  from  one  border  to  the  other,  and  7000  years  to 
reach  us  from  the  extreme  boundary. 


RESEARVHES  OF  HERSCHEL  AND  HIS  SUCCESSOUS.     469 


The  foregoing  deduction  of 
Ilerschel  was  founded  on  the 
hypothesis  that  the  stars  were 
equally  dense  in  every  part  of 
the  stellar  system,  so  that  the 
number  of  stars  in  any  direc- 
tion furnished  an  index  to  the 
extent  of  the  stars  in  that  di- 
rection. Further  study  show- 
ed Ilerschel  that  this  assump- 
tion might  be  so  far  from  cor- 
rect that  his  conclusions  would 
have  to  be  essentially  modi- 
fied. Binary  and  other  double 
stars  and  star  clustei-s  evident- 
ly offered  ^ases  in  which  sev- 
eral stars  were  in  much  closer 
association  than  were  the  stars 
in  general.  To  show  exactly 
on  what  considerations  this 
change  of  view  is  founded,  wo 
remark  that  if  the  increase  of 
density  in  the  direction  of  the 
Milky  Way  were  quite  regu- 
lar, so  that  there  were  no  cases 
of  great  difference  in  the  thick- 
ness of  the  stars  in  two  adjoin- 
ing regions,  then  the  original 
view  would  have  been  sound 
so  far  as  it  went.  But  such  ir- 
regularities a^e  very  frequent, 
and  it  would  lead  to  an  obvi- 
ous absurdity  to  explain  them 
on  Ilerschel's  first  hypothesis ; 
for  instance,  when  the  tele- 
scope was  directed  towards 
the  Pleiades  there  would  be  f»o-io9. 


-Herschel's  view  of  the  form  of  the 
universe. 


470  THE  STELLAR  UNIVERSE. 

found,  prol)ab]y,  six  or  eight  times  as  many  stare  as  in  the  ad- 
joining fields.  But  supposing  the  real  thickness  of  the  stars 
the  same,  the  result  would  be  that  in  this  particular  direction 
the  stars  extended  out  twice  as  far  as  they  did  in  the  neigh- 
boring parts  of  the  sky;  that  is,  we  should  have  a  long,  nar- 
row spike  of  stars  pointing  directly  from  us.  As  there  are 
many  such  clusters  in  various  parts  of  the  sky,  we  should  have 
to  suppose  a  great  number  of  such  sp"kes.  In  other  regions, 
especially  around  the  Milky  Way,  there  are  spaces  nearly  void 
of  stars.  To  account  for  these  we  should  have  to  suppose 
long  narrow  chasms  reaching  through  towards  our  sun.  Thus 
the  stellar  system  would  present  the  form  of  an  exaggerated 
star-fish  with  numerous  deep  openings,  a  form  the  existence 
of  which  is  beyond  all  probability,  especially  if  we  reflect 
that  all  the  openings  and  all  the  arms  liave  to  proceed  from 
the  direction  of  our  sun. 

The  only  rational  explanation  of  a  group  of  stars  showing 
itself  in  a  telescope,  with  a  comparatively  void  space  surround- 
ing it,  is  that  we  have  here  a  real  star  cluster,  or  a  region  in 
which  the  stars  are  thicker  than  elsewhere.  Now,  one  can  see 
with  the  naked  eye  that  the  Milky  Way  is  not  a  continuous 
uniform  belt,  but  is,  through  much  of  its  course,  partly  made 
up  of  a  great  number  of  irregular  cloud-like  masses  with  com- 
paratively dark  spaces  between  them.  The  conclusion  is  un- 
avoidable that  we  have  here  real  aggregations  of  stars,  and 
not  merely  a  region  in  which  the  bounds  of  the  stellar-sys- 
tem are  more  widely  extended.  Whether  Ilerschel  clearly  saw 
this  may  be  seriously  questioned ;  but  however  it  may  have 
been,  he  adopted  another  method  of  estimating  the  relative 
distances  of  the  stars  visible  in  his  gauges. 

This  method  consisted  in  judging  of  the  distances  to  which 
his  telescope  penetrated,  not  by  the  number  of  stars  it  brought 
into  view,  but  by  their  brightness.  If  all  the  stars  were  of  the 
same  intrinsic  brightness,  so  that  the  differences  of  their  ap- 
parent magnitude  arose  only  from  their  various  distances  from 
us,  then  this  method  would  enable  us  to  fix  the  distance  of 
each  separate  star.     Eut  as  we  know  that  the  stars  are  by  no 


RESEARCHES  OF  EERSCHEL  AXD  HIS  SUCCESSORS.    471 


means  equal  in  intrinsic  brightness,  the  method  cannot  be 
safely  applied  to  any  individual  star,  a  fact  which  llerschel 
himself  clearly  saw.  It  does  not  follow,  however,  that  we 
cannot  thus  form  an  idea  of  the  relr.tive  distances  of  whole 
classes  or  groups  of  stars.  Although  it  is  quite  possible  that 
an  individual  star  of  the  fifth  magnitude  may  be  nearer  to  us 
than  another  of  the  fourth,  yet  we  cannot  doubt  that  the  av- 
erage distance  of  all  the  fifth-magnitude  stars  is  greater  than 
the  average  of  those  of  the  fourth  magi  itude,  and  greater, 
too,  in  a  proportion  admitting  of  a  toleraljly  accurate  numeri- 
cal estimate.  Such  an  estimate  llerschel  attempted  to  make, 
proceeding  on  the  following  plan  : 

Suppose  a  spliere  to  be  drawn  around  our  sun  as  a  centre 
of  such  size  that  it  shall  be 
equal  to  the  average  space 
occupied  by  a  single  one  of 
the  stars  visible  to  the  naked 
eye;  that  is,  if  we  suppose 
that  portion  of  the  space  of 
the  stellar  system  occupied 
by  the  six  thousand  bright- 
er stars  to  be  divided  into 
six  thousand  parts,  then  the 
sphere  wnll  be  equal  to  one 
ot  these  parts.  The  radius 
of  this  spliere  will  probably 
not  differ  much  from  the  dis- 
tance of  the  nearest  lixed  star, 
a  distance  we  shall  take  for 
unity.  Then,  suppose  a  series 
of  larger  spheres,  all  drawn 
around  our  sun  as  a  centre, 
and  having  the  radii  3,  5,  7, 
9,  etc.  The  contents  of  the 
spheres  being  as  the  cubes 
of  their  diameters,  the  first 
sphere  will  have  3  x  3  x  3 =27 


Fig.  110.— Illustrating  Herschel's  orders  of  dis- 
tauce  of  the  stars. 


472 


THE  STELLAR  UNIVERSE. 


times  the  bulk  of  the  unit  sphere,  and  will  therefore  be  large 
enough  to  contain  27  stars ;  the  second  will  have  125  times 
the  bulk,  and  will  therefore  contain  125  stars,  and  so  with 
the  successive  spheres.  Fig.  130  shows  a  section  of  portions 
of  these  spheres  up  to  that  with  radius  11.  Above  the  centre 
are  given  the  various  orders  of  stars  which  are  situated  be- 
tween the  several  spheres,  while  in  the  corresponding  spaces 
below  the  centre  are  given  the  number  of  stars  which  the  re- 
gion is  large  enough  to  contain ;  for  instance,  the  sphere  of 
radius  7  has  room  for  343  stars,  but  of  this  space  125  parts 
belong  to  the  spheres  inside  of  it :  there  is,  therefore,  room  for 
218  stars  between  the  spheres  of  radii  5  and  7. 

Ilerschel  designates  the  several  distances  of  these  layers  of 
stars  as  orders ;  the  stars  between  s])heres  1  and  3  are  of  the 
first  order  of  distance,  those  between  3  and  5  of  the  second 
order,  and  so  on.  Comparing  the  room  for  stai-s  between  the 
several  spheres  with  the  number  of  stars  of  the  several  magni- 
tudes, he  found  the  result  to  be  as  follows : 


Order  of 
Distance. 

In  umber  of 

Number  of 

Stars  there 
is  room  for. 

Magnitiulj. 

Stars  of  that 
inat^nituile. 

1 

26 

1 

17 

2 

98 

2 

57 

3 

218 

3 

206 

4 

386 

4 

454 

5 

602 

5 

1161 

6 

866 

6 

♦3103 

7 

1178 

7 

6146 

8 

1038 

There  is  evidently  no  correspondence  between  the  calculat- 
ed orders  of  distance  and  the  magnitudes  as  estimated  on  the 
usual  scale.  But  Ilerschel  found  that  this  was  because  the 
magnitudes  as  usually  estimated  corresponded  to  an  entheiy 
different  scale  of  distance  from  that  which  he  adopted.  In 
his  scale  the  several  distances  increased  in  arithmetical  pro- 
gression ;  while  in  the  order  of  magnitudes  the  increase  is 
in  geometrical  progression.  In  consequence,  the  stars  of  the 
sixth  magnitude  correspond  to  the  eighth,  ninth,  or  tenth  order 
of  distances ;  that  is,  we  should  have  to  remove  a  star  of  the 


BESE ARCHES  OF  HEESCHEL  AND  HIS  SUCCESSOIiS.     473 

first  magnitude  to  eight,  nine,  or  ten  times  its  actual  distance 
to  make  it  shine  as  a  star  of  the  sixth  magnitude. 

Attempting  on  this  system  to  measure  the  extent  of  the 
Milky  Way,  Ilerschel  concluded  that  it  was  unfathomable 
with  his  twenty -foot  telescope,  which,  he  calculated,  would 
penetrate  to  tlie  900th  order  of  distances,  that  is,  to  stars 
which  were  900  times  as  far  as  the  average  of  those  of  the 
first  magnitude.  He  does  not  seem  to  have  made  any  very 
extended  examination  with  his  forty-foot  telescope,  but  con- 
cluded that  it  would  leave  him  in  the  same  uncertainty  in 
respect  to  the  extent  of  the  Milky  Way  as  the  twenty-foot  one 
did.  This  unrivalled  man.  to  whom  it  was  given  to  penetrate 
farther  into  creation  than  man  had  ever  done  before  him, 
seems  to  have  rested  from  his  labors  without  leaving  any  more 
definite  theory  of  the  boundaries  of  the  stellar  system  than 
that  they  extended,  at  least  in  the  direction  of  the  Milky  Way, 
beyond  the  utmost  limit  to  which  his  telescope  could  penetrate. 
If  we  estimate  the  time  it  would  require  light  to  come  from 
the  utmost  limit  to  which  he  believed  his  vision  to  extend, 
we  shall  find  it  to  be  about  fourteen  thousand  yeai-s,  or  more 
than  double  that  deduced  from  his  former  gauges.  We  can 
say  with  confidence  that  the  time  required  for  light  to  reach 
us  from  the  most  distant  visible  stars  is  measured  by  thou- 
sands of  years.  But  it  must  be  admitted  that  Ilerschel's  esti- 
mate of  the  extent  of  the  Milky  Way  may  be  far  too  great,  be- 
cause it  rests  on  the  assumption  that  all  stars  are  of  the  same 
absolute  brightness.  If  the  smallest  stars  visible  in  his  tele- 
scope were,  on  the  average,  of  the  same  intrinsic  brilliancy  as 
the  brighter  ones,  the  conclusion  would  be  well  founded.  But 
if  we  suppose  a  boundary,  it  is  impossible  to  decide  from  Iler- 
schel's data  whether  the  minuteness  of  those  stars  arises  from 
their  great  distance  or  from  their  small  magnitude.  Notwith- 
standing this  uncertainty,  it  has  been  maintained  by  some,  not- 
ably by  Mr.  Proctor,  that  the  views  of  Ilerschel  respecting  the 
constitution  of  the  Milky  Way,  or  stellar  system,  were  radical- 
ly changed  by  this  second  method  of  star-gauging.  I  see  no 
evidence  of  any  radical  change.     Although  Ilerschel  does  not 


474  THE  STELLAR  UNIVERSE. 

express  himself  very  definitely  on  the  subject,  yet,  in  his  last 
paper  on  the  distribution  of  the  stars  {Philosophical  Trans- 
actions  for  1817),  tliere  are  several  remarks  which  seem  to  im- 
ply that  he  still  supposed  the  stellar  system  to  have  the  gen- 
eral fc  shown  in  Fig.  109,  and  that,  in  accordance  witli  that 
view,  he  supposed  the  clustering  of  stars  to  indicate  protuber- 
ant parts  of  the  Milky  Way.  He  did,  indeed,  apply  a  differ- 
ent method  of  research,  but  the  results  to  which  the  new  meth- 
ods led  were,  in  their  main  features,  the  same  as  those  of  the 
old  metliod. 

Since  the  time  of  Ilerschel,  one  of  the  most  eminent  of  the 
astronomers  who  have  investigated  this  subject  is  Struve  the 
elder,  formerly  director  of  tlie  Pulkowa  Observatory.  His  re- 
searches were  founded  mainly  on  the  numbers  of  stars  of  the 
several  magnitudes  found  by  Bessel  in  a  zone  thirty  degrees 
wide  extending  all  round  the  heavens,  fifteen  degrees  on  each 
side  of  the  equator.  With  these  he  combined  the  gauges  of 
Sir  William  Herschel.  The  hypothesis  on  which  he  based  his 
theory  was  similar  to  that  employed  by  Herschel  in  his  later 
researches,  in  so  far  that  he  supposed  the  magnitude  of  the 
stars  to  furnish,  on  the  average,  a  measure  of  tlieir  relative 
distances.  Supposing,  after  Herschel,  a  number  of  concentric 
spheres  to  be  drawn  around  the  sun  as  a  centre,  the  successive 
spaces  between  wliich  corresponded  t<  stars  of  the  several 
magnitudes,  he  found  that  the  farther  out  he  went,  the  more 
the  stars  were  condensed  in  and  near  the  Milky  Way.  This 
conclusion  may  be  drawn  at  once  from  the  fact  we  have  al- 
ready mentioned,  tliat  the  smaller  the  stars,  the  more  they  are 
condensed  in  the  region  of  the  Galaxy.  Struve  found  that  if 
we  take  only  the  stais  plainly  visible  to  the  naked  eye — that 
is,  those  down  to  the  fifth  magnitude — they  are  no  thicker  in 
the  Milky  Way  than  in  other  parts  of  tlie  heavens.  But  those 
of  tlie  sixth  magnitude  are  a  little  tliicker  in  that  region,  those 
of  the  seventh  yet  thicker,  and  so  on,  the  inequality  of  distri- 
bution becoming  constantly  greater  as  the  telescopic  power  is 
increased. 

From  all  this,  Struve  concluded  that  the  stellar  system  might 


EESEABCHES  OF  HERSCHEL  AND  HIS  SUCCESSORS.     475 

be  considered  as  composed  of  laj^ers  of  stars  of  various  densi- 
ties, all  paniUel  to  the  plane  of  the  Milky  Way.  The  stars  are 
thickest  in  and  near  the  central  layer,  which  he  conceives  to 
be  spread  out  as  a  wide,  thin  sheet  of  stars.  Our  sun  is  situ- 
ated near  the  middle  of  this  layer.  As  we  pass  out  of  this 
layer,  on  either  side  we  find  the  stars  constantly  growing  thin- 
ner and  thinner,  but  we  do  not  reach  any  distinct  boundary. 
As,  if  we  could  rise  in  the  atmosphere,  we  should  find  the  air 
constantly  growing  thinner,  but  at  so  gradual  a  rate  of  prog- 
ress that  we  could  hardly  say  where  it  terminated ;  so,  on 
Struve's  view,  would  it  be  with  the  stellar  system,  if  wc  could 
mount  up  in  a  direction  perpendicular  to  the  Milky  Way. 
Struve  gives  the  following  table  of  the  thickness  of  the  stars 
on  each  side  of  the  principal  plane,  the  unit  of  distance  being 
that  of  the  extreme  distance  to  which  Ilerschel's  telescope 
could  penetrate : 


Mean  Distance 

Distance  from  Principiil  Flane. 

Density. 

between  Neigh- 
borint;  Stars. 

In  tlie  priiici|)al  pliine 

1.0000 

0.48508 

1.000 
1.272 

0.05  from  piiucipal  phuie 

0.10          "              "          

0.3:?'-'H8 

1.458 

0.20          "              "          

0.2:5895 

1.011 

O.JJO          "              "          

0.171»80 

1.772 

0.40          "              "          

0. 11502 1 

1.073 

o.m         "             "         

0.08040 

2.201 

O.GO          "              "          

0.05510 

2.028 

0.70          "              "          

0.03071) 

3.1il0 

0.80           "               "          

0.01414 

4. 131 

0.80(5         "               "          

0.00532 

5.72!) 

This  condensation  of  the  stars  near  the  central  plane,  and 
the  gradual  thinning-out  on  each  side  of  it,  are  only  designed 
to  be  the  expression  of  the  general  or  average  distribution 
of  those  bodies.  The  probability  is  that  even  in  the  central 
plane  the  stars  are  many  times  as  thick  in  some  regions  as  in 
others,  and  that  as  we  leave  the  plane,  the  thinning-out  would 
be  found  to  proceed  at  very  different  rates  in  different  re- 
gion; That  there  may  be  a  gradual  thinning- out  cannot  be 
denied ;  but  Struve's  attempt  to  form  a  table  of  it  is  open  to 
the  serious  objection  tliat,  like  Ilerschcl,  he  supposed  the  dif- 


476  THE  STELLAR   UNIVERSE. 

ferences  between  the  magnitudes  of  the  stars  to  arise  entirely 
from  their  different  distances  from  us.  Although  where  the 
scattering  of  the  stars  is  nearly  uniform  this  supposition  may 
not  lead  us  into  serious  erroi-,  the  case  will  be  entirely  differ- 
ent where  we  have  to  deal  with  irregular  masses  of  stars,  and 
especially  where  our  telescopes  penetrate  to  the  boundar}-  of 
the  stellar  system.  In  the  latter  case  we  cannot  possibly  dis- 
tinguish between  small  stars  lying  within  the  boundary  and 
larger  ones  scattered  outside  of  it,  and  Struve's  gradual  thin- 
ning-out of  the  stars  mav  be  entirely  accounted  for  by  great 
diversitie     n  the  absolute  brightness  of  the  stars. 

Among  recent  researches  on  this  subject,  those  of  Mr.  R. 
A.  Proctor  are  entitled  to  consideration,  from  being  founded 
on  facts  which  were  not  fully  known  or  understood  by  the 
investigators  whom  we  have  mentioned.  The  strongest  point 
which  he  makes  is  that  all  views  of  the  arrangement  of  the 
stellar  system  founded  upon  the  theory  that  the  stars  are 
either  of  similar  intrinsic  brightness,  or  approach  an  equality 
of  distribution  in  different  regions,  are  entirely  illusory.  He 
cites  the  phenomena  of  star-drift,  described  in  the  last  chap- 
ter, as  proving  that  stars  which  had  been  supposed  widely  sep- 
arated are  really  agglomerated  into  systems;  and  claims  that 
the  Milky  Way  may  be  a  collection  of  such  systems,  having 
nothing  like  the  extent  assigned  it  by  Ilerschel. 

How  far  the  considerations  brought  forward  by  Mr.  Proc- 
tor should  make  us  modify  the  views  of  the  subject  hitherto 
held,  cannot  be  determined  without  further  observations  on  the 
clustering  of  stars  of  different  magnitudes.  We  may,  howev- 
er, safely  concede  that  there  is  a  greater  tendency  among  the 
stars  to  be  collected  into  groups  than  was  formerly  supposed. 
A  curious  result  of  Mr.  J.  M.  Wilson,  of  Rugby,  England,  re- 
specting the  orbits  of  some  binary  stars,  throws  light  on  this 
tendency.  It  was  found  by  Struve  that  although  the  great 
common  proper  motion  of  the  pair  of  stars  61  Cygni,  cele- 
brated for  the  determinations  of  their  parallax,  was  such  as  to 
leave  no  reasonable  doubt  that  they  were  physically  connect- 
ed, yet  not  the  slightest  deviation  in  their  courses,  arising 


RESEARCHES  OF  HERSCUEL  AND  HIS  SUCCESS*' "iS.     477 

from  thcii"  mutual  attraction,  conld  be  detected.  Mr.  Wilson 
has  recently  confirmed  this  result  by  an  examination  of  the 
whole  series  of  measures  on  this  pair  from  1753  to  1S74, 
which  do  not  show  the  slightest  deviation,  but  seem  to  indi- 
cate that  each  star  of  the  pair  is  going  on  its  course  indepen- 
dently of  the  other.  But,  as  just  stated,  they  move  too  nearly 
together  to  permit  of  the  belief  that  they  are  really  indepen- 
dent. The  only  conclusion  open  to  us  is  that  each  of  them  de- 
scribes an  immense  orbit  around  their  common  centre  of  grav- 
ity, an  orbit  which  may  be  several  degrees  in  apparent  diam- 
eter, and  in  whicli  the  time  of  revolution  is  counted  by  thou- 
sands of  years.  Two  thousand  years  hence  they  will  be  so 
far  apart  that  no  connection  between  them  would  be  sus- 
pected. 

It  is  a  question  whether  we  have  not  another  instance  of 
the  same  kind  in  the  double  star  Castor,  or  a  Geminorum. 
Mr.  Wilson  finds  the  orbit  of  this  binary  to  be  apparently 
hyperbolic,  a  state  of  things  which  would  indicate  that  the 
two  stars  had  no  physical  connection  whatever,  but  that,  in 
pui*suing  their  courses  through  space,  they  chanced  to  come 
so  close  together  that  they  were  brought  for  a  while  within 
each  other's  sphere  of  attraction.  If  this  be  the  case,  they 
will  gradually  separate  forever,  like  two  ships  meeting  on  the 
ocean  and  parting  again.  We  remark  that  the  course  of  each 
star  will  then  be  very  different  from  what  it  would  have 
been  if  they  had  not  met.  We  cannot,  however,  accept  the 
hyperbolic  orbit  of  Mr.  Wilson  as  an  established  fact,  because 
the  case  is  one  in  which  it  is  very  difficult  to  distinguish  be- 
tween a  large  and  elongated  elliptic  orbit  and  a  hyperbolic 
orbit.  The  common  proper  motion  of  the  two  objects  is  such 
as  to  lead  to  the  belief  that  they  constitute  a  pair,  the  compo- 
nents of  which  separate  to  a  great  distance. 

Now,  these  discoveries  of  pairs  of  stai*s  moving  around  a 
common  centre  of  gravity,  in  orbits  of  innnense  extent,  sug- 
gest the  probability  that  there  exist  in  the  heavens  great  num- 
bers of  pail's,  clusters,  and  systems  of  this  sort,  the  members 
of  which  are  so  widely  separated  that  they  have  never  been 


478  THE  STELLAR   UNIVERSE. 

suspected  to  belong  together,  and  the  widely  scattered  groups 
having  a  common  proper  motion  may  very  well  be  systems  of 
this  kind. 

§  3.  Probable  Arrangement  of  the  Visible  Universe. 

The  preceding  description  of  the  views  held  l)y  several  gen- 
erations of  profound  thinkers  and  observers  resi)ecting  the 
arrangement  of  the  visible  universe  furnishes  an  example  of 
what  we  may  call  the  evolution  of  scientific  knowledge.  Of 
no  one  of  the  great  men  whom  we  have  mentioned  can  it  be 
said  that  his  views  were  absolutely  and  unqualifiedly  errone- 
ous, and  of  none  can  it  be  said  that  he  reiched  vlie  entire 
truth.  Their  attempts  to  solve  the  mystery  which  they  saw 
before  them  werp  like  those  of  a  spectator  to  make  out  the  ex- 
act structure  of  a  great  building  which  he  sees  at  a  distance 
in  the  dim  twilight.  He  first  sees  that  the  building  is  really 
there,  and  sketches  out  what  he  believes  to  be  its  outlines.  As 
the  light  increases,  he  finds  that  his  first  outline  bears  but  a 
rude  resemblance  to  what  now  seems  to  be  the  real  form,  and 
he  corrects  it  accordingly.  In  his  first  attempts  to  fill  in  the 
columns,  pilasters,  windows,  and  doors,  he  mistakes  the  darker 
shades  between  the  columns  for  windows,  other  lighter  shad- 
ows for  doors,  and  the  pilasters  for  columns.  Notwithstand- 
ing such  mistakes,  his  representation  is  to  a  certain  extent  cor- 
rect, and  he  will  seldom  fall  into  egregious  error.  The  suc- 
cessive improvements  in  his  sketch,  from  the  first  rough  out- 
line to  the  finished  picture,  do  not  consist  in  effacing  at  each 
step  everything  h*^.  has  done,  but  in  correcting  it,  and  filling  in 
the  details. 

The  progress  of  our  knowledge  of  nature  is  generally  of  this 
character.  But  in  the  case  now  before  us,  so  great  is  the  dis- 
tance, so  dim  the  light,  and  so  slender  our  ideas  of  the  princi- 
ples on  which  the  vast  fabric  is  constructed,  that  we  cannot 
pass  beyond  a  few  rough  outlines.  Still  there  are  a  few  feat- 
ures which  we  can  describe  with  a  near  ajiproach  to  certainty, 
and  others  respecting  which,  though  our  knowledge  is  some- 
what vague,  we  can  reach  a  greater  or  less  degree  of  proba- 


PROBABLE  ARRANGEMENT  OF  THE  VISIBLE  UNIVERSE.  479 

bility.    "We   may   include  these  under  the  following  seven 
heads : 

1st.  Leaving  the  nebulae  out  of  consideration,  and  confining 
ourselves  to  the  stellar  ej-steni,  we  may  say,  with  moral  cer- 
tainty, that  the  great  mass  of  the  stars  which  compose  this 
system  are  spread  out  on  all  sides,  in  or  near  a  widely  extend- 
ed plane  passing  through  the  Milky  Way.  In  other  words, 
the  large  majority  of  the  stars  which  we  can  see  with  the  tele- 
scope are  contained  in  a  space  having  the  form  of  a  round,  flat 
disk,  the  diameter  of  which  is  eight  or  ten  times  its  thickness. 
This  was  clearly  seen  by  Kant,  and  has  been  confirmed  by 
Ilerschel  and  Struve.  In  fact,  it  forms  the  fundamental  base 
of  the  structures  reared  by  these  several  investigators.  When 
Kant  saw,  in  this  arrangement,  a  resemblance  to  the  solar 
system,  in  which  the  planets  all  move  round  near  one  central 
plane,  he  was  correct,  so  far  as  he  went.  The  space,  then,  in 
which  we  find  most  of  the  stars  to  be  contained  is  bounded 
by  two  parallel  planes  forming  the  upper  and  lower  surfaces 
of  the  disk  we  have  described,  the  distance  apart  of  these 
planes  being  a  small  fraction  of  their  extent  —  probably  less 
than  an  eighth. 

2d.  Within  the  space  we  have  described  the  stars  are  not 
scattered  uniformly,  but  are  for  the  most  part  collected  into 
irregular  clusters  or  masses,  with  comparatively  vacant  spaces 
between  them.  These  collections  have  generally  no  definite 
boundaries,  but  run  into  each  other  by  insensible  gradations. 
The  number  of  stars  in  each  collection  may  range  from  two 
to  many  thousands ;  and  larger  masses  are  made  up  of  smaller 
ones  in  every  proportion,  much  as  the  heavy  clouds  on  a  sum- 
mer's day  are  piled  upon  each  other. 

3d.  Our  sun,  with  its  attendant  planets,  is  situated  near  the 
centre  of  the  space  we  have  described,  so  that  we  see  nearly 
the  same  number  of  stars  in  any  two  opposite  quarters  of  the 
heavens. 

4th.  The  six  or  seven  thousand  stars  around  us,  which  are 
easily  seen  by  the  naked  eye,  are  scattered  in  space  with  a 
near  approach  to  uniformity,  the  only  exception  being  local 


480  THE  STELLAR   UNIVERSE. 

clusters,  the  component  stars  of  wliicli  are  few  in  number  and 
pretty  widely  separated.  Such  are  the  Pleiades,  Coma  Bere- 
nices, and  perhaps  tlie  principal  stars  of  many  other  constella- 
tions, wliicli  are  so  widely  separated  that  we  do  not  see  any 
connection  amonjj  them. 

5th.  The  disk  which  we  have  described  does  not  represent 
the  form  of  the  stellar  system,  but?  only  the  limits  within 
which  it  is  mostly  contained.  The  absence  of  any  definite 
boundary,  either  to  star  clusters  or  the  stellar  system,  and  the 
number  of  comparatively  vacant  regions  here  and  there  among 
the  clusters,  prevent  our  assigning  any  more  definite  form  to 
the  system  tlian  we  could  assign  to  a  cloud  of  dust.  The  thin 
and  widely  extended  space  in  which  the  stars  are  most  thickly 
clustered  may,  however,  be  called  the  galactic  region. 

6th.  On  each  side  of  the  galactic  region  the  stars  are  more 
evenly  and  thinly  scattered,  but  probably  do  not  extend  out  to 
a  distance  at  all  approaching  the  extent  of  the  galactic  region. 
If  they  do  extend  out  to  an  equal  distance,  they  are  very  few 
in  number.  It  is,  however,  impossible  to  set  any  definite  boun- 
daries, not  only  from  our  ignorance  of  the  exact  distance  of 
the  smallest  stars  we  can  see  in  the  telescope,  but  because  the 
density  of  the  stars  probably  diminishes  very  gradually  as  we 
go  out  towards  the  boundary. 

7th.  On  each  side  of  the  galactic  and  stellar  region  we  have 
a  nebular  region,  in  which  we  find  few  or  no  stars,  but  vast 
numbers  of  nebulae.  The  nebulae  diminish  greatly  in  num- 
ber as  we  approach  the  galactic  region,  only  a  very  few  being 
found  in  that  region. 

The  general  arrangement  of  the  stars  and  nebulte  which  we 
have  described  is  seen  in  Fig.  Ill,  which  shows  what  is  prob- 
ably the  general  aspect  of  a  section  of  the  visible  universe  per- 
pendicular to  the  Milky  Way.  In  the  central  part  of  the  fig- 
ure we  have  the  galactic  region,  in  which  the  stars  are  mostly 
ajvrrreo-ated  in  laro-e  masses.  Of  the  arrangement  of  these 
masses  nothing  certain  is  known ;  they  are,  therefore,  put  in 
nearly  at  random.  Indeed,  it  is  still  an  undecided  question 
whether  the  aggregations  of  stars  which  make  up  the  Milky 


PBOBABLE  AltRANGEMENT  OF  THE  VISIBLE  UXIFEBSE.   481 

Way  extend  all  the  way  across  the  dia»netcr  of  the  galactic 
region,  or  whether  they  are  arranged  in  the  form  of  a  ring, 
with  our  sun  jand  his  surrounding  stars  in  the  centre  of  it. 
In  the  latter  case,  the  masses  of  stars  near  the  centre  should 
l)e  loss  strongly  marked.  This  central  region  heing  that  in 
which  our  earth  is  situated,  this  nncertainty  respecting  the 
density  of  stars  in  that  region  implies  an  uncertainty  whether 


Fig.  111. — Probable  nrrangement  of  the  stars  and  uebiihe  visible  with  the  telescope.    la 
the  Galaxy  the  stars  are  not  evenly  scattered,  but  arc  agglomerated  into  clusters. 

the  stars  visible  with  the  naked  eye  are  part  of  one  of  the 
masses  which  make  nj)  the  Galaxy,  or  whether  we  are  in  a 
comparatively  thin  region.  Although  this  question  is  still 
unsolved,  it  is  one  which  admits  of  an  answer  by  telescopic 
research.  When  we  described  Sir  William  Ilerschers  ar- 
rangement of  the  stars  in  concentric  spheres,  we  saw  that  in 
the  more  distant  spheres  the  stars  were  vastly  more  dense 

32 


482  THE  STELLAR   UNIVERSE. 

around  the  galactic  belt  of  each  sphere  than  they  were  in 
other  parts  of  it.  To  answer  the  question  which  has  been 
presented,  we  must  compare  the  densities  of  the  stars  at  the 
circumferences  of  these  spheres  with  the  density  immediately 
around  us.  In  other  words,  the  question  is,  Suppose  a  human 
being  could  dart  out  in  the  direction  of  the  Milky  Way,  and 
pass  through  some  of  the  masses  of  stars  composing  it,  would 
he  find  them  thicker  or  thinner  than  they  are  in  the  visible 
heavens  around  us  ? 

A  question  still  left  open  is,  whether  all  the  celestial  objects 
visible  with  the  telescope  are  included  within  the  limits  of  the 
three  regions  we  have  just  indicated,  or  whether  the  whole 
Galaxy,  with  everything  which  is  included  within  its  limits, 
is  simply  one  of  a  great  number  of  widely  scattered  stellar 
systems.  Since  any  consideration  of  invisible  galaxies  and 
systems  would  be  entirely  idle,  the  question  may  be  reduced 
to  this :  Are  the  most  distant  star  clusters  which  the  telescope 
shows  us  situated  within  the  limits  of  the  stellar  system  or  far 
without  them,  a  great  vacant  space  intervening?  The  latter 
alternative  is  the  popular  one,  first  suggested  by  Kant,  it  be- 
ing supposed  that  the  most  distant  nel)ula3  constituted  other 
Milky  Ways  or  stellar  systems  as  extensive  as  our  own. 

Although  the  possibility  that  this  view  is  correct  cannot  be 
denied,  yet  the  arrangement  of  the  star  clusters  or  resolvable 
nebula?  militates  against  it.  We  have  shown  that  the  major- 
ity of  the  latter  lie  near  the  direction  of  the  plane  of  the 
Milky  Way,  comparatively  few  being  seen  near  the  perpen- 
dicular direction.  But  if  these  objects  were  other  galaxies, 
far  outside  of  the  one  which  surrounds  us,  they  would  be  as 
likely  to  lie  in  one  direction  as  in  another,  and  the  probabil- 
ity against  the  great  mass  of  them  lying  in  one  plane  would 
be  very  great.  The  most  probable  conclusion,  therefore,  is 
that  they  constitute  part  of  our  stellar  system.  They  may,  in- 
deed, be  scattered  around  or  outside  of  the  extreme  limits  with- 
in which  single  stars  can  be  seen,  but  not  at  distances  so  great 
that  they  should  be  considered  as  separate  systems.  The  most 
probable  conclusion,  in  the  present  state  of  our  knowledge, 


DO   THE  STJllS  REALLY  FORM  A   SYSTEM f  483 

seeins  to  be  that  the  scheme  shown  in  Fig.  Ill  indudcs  the 
wliole  visible  universe. 

The  differences  of  opinion  which  now  exist  respecting  the 
probable  arrangement  and  distance  of  the  stars  arise  mainly 
from  our  nncertainty  as  to  what  is  the  probable  range  of  ab- 
solute magnitude  of  the  stars,  a  subject  to  wlii  \  we  have  al- 
ready several  times  alluded.  The  discovery  of  the  parallax 
of  several  stars  has  enabled  us  not  only  to  form  some  idea  of 
this  question  by  comparing  the  brillian(!y  of  these  stars  witli 
their  known  distances,  but  it  has  enabled  us  to  answer  the  in- 
teresting question,  How  does  our  sun  compare  with  these  stars 
in  brightness  ?  The  curious  result  of  this  in(juiry  is,  that  our 
sun  is  really  a  star  less  than  the  average,  which  would  mod- 
estly twinkle  among  the  snialler  of  its  fellows  if  removed 
to  the  distance  from  us  at  which  they  are  placed.  Zullner 
found,  by  comparing  the  light  of  the  sun  with  that  of  Capella, 
or  a  AurigiiB,  that  it  would  have  to  be  removed  to  230,000 
times  its  present  distance  to  appear  equally  bright  with  that 
star,  which  we  may  take  as  an  average  star  of  the  first  magni- 
tude. But  the  greater  number  of  the  stars  of  this  magnitude 
are  situated  at  four  or  five  times  this  distance ;  so  that  if  our 
sun  were  placed  at  their  average  distance,  it  would  probably 
not  exceed  the  third  or  fourth  magnitude.  Still,  it  would  by 
no  means  belong  among  the  smallest  stars  of  all,  because  we 
do  find  stars  with  a  measurable  parallax  which  are  only  of 
the  fifth,  sixth,  or  even  the  seventh  magnitude.  Altogether,  it 
appears  that  the  range  of  absolute  brilliancy  among  the  stars 
extends  through  eight  or  ten  magnitudes,  and  that  the  largest 
ones  emit  several  thousand  times  as  much  light  as  the  small- 
est. It  is  this  range  of  magnitude  which  really  forms  the 
greatest  obstacle  in  the  way  of  determining  the  arrangement 
of  the  stars  in  space. 

§  4.  Do  the  Stars  recdh/foi'm  a  System? 

We  have  described  the  sublime  ideas  of  Kant  and  Lam- 
bert, who,  seeing  the  l)odies  of  our  solar  system  fitted  to  go 
through  their  revolutions  without  permanent  change  during 


484  THE  STELLAR   UNIVERSE. 

an  indefinite  period  of  time,  reasoned  by  analogy  that  the 
stellar  universe  was  constructed  on  the  same  general  })lan, 
and  that  each  star  had  its  appointed  orbit,  round  which  it 
would  run  its  coui*se  during  endless  ages.  This  speculation 
was  not  followed  up  by  Ilerschel  and  Struve,  wh  proceeding 
on  a  more  strictly  scientilic  plan,  found  it  necessary  to  learn 
how  the  stars  are  now  situated  before  attempting  to  decide 
in  what  kinds  of  orbits  they  are  moving.  In  the  absence  of 
exact  knowledge  respecting  the  structure  and  extent  of  the 
stellar  system,  it  is  impossible  to  say  with  certainty  what  will 
be  the  state  of  that  system  after  the  lapse  of  the  millions  of 
years  which  would  be  necessary  for  the  stars  to  perform  a 
revolution  around  one  centre.  I>ut,  as  in  describing  the  con- 
stitution of  the  stellar  system,  we  found  certain  features  on 
which  we  could  pronounce  with  a  high  degree  of  probability, 
so,  in  respect  to  the  motions  and  orbits  of  tlie  stars,  there  are 
-ome  propositions  which  we  may  sustain  with  a  near  approach 
to  certainty. 

Stability  of  the  System. — "We  may  first  assert,  with  a  high  de- 
gree of  probability,  that  the  stars  do  not  form  a  stable  system 
in  the  sense  in  which  we  say  that  the  solar  system  is  stable. 
By  a  stable  system  we  mean  one  in  which  each  star  moves 
round  and  round  in  an  unclianging  orbit,  every  revolution 
bringing  it  back  to  its  starting-point,  so  that  the  system  as  a 
whole  shall  retain  the  same  general  form,  dimensions,  and 
arrangement  during  iimumerable  revolutions  of  the  bodies 
which  compose  it.  It  is  almost  necessary  to  the  existence  of 
such  a  system  that  it  have  a  great  central  body,  the  mass  of 
which  should  be  at  least  vastly  greater  than  that  of  the  indi- 
vidual bodies  which  revolve  around  it.  At  least,  such  a  cen- 
tral body  could  be  dispensed  with  only  by  the  separate  stars 
having  a  regularity  of  motion  and  arrangement  which  cer- 
tainly does  not  exist  in  the  stellar  system  as  we  actually  see 
it.  The  question,  then,  reduces  itself  to  this:  Are  there  any 
immense  attracting  centres  around  which  the  separate  collec- 
tions of  stars  revolve ;  or  is  there  any  centre  around  whicii  all 
tlie  stars  which  compose  the  visible  universe  revolve  ?     In  all 


DO   THE  STABS  REALLY  FORM  A   SYSTEM f  485 

huinan  probability^  these  qnestioiis  must  be  answered  in  tlie 
negative.  All  analogy  leads  us  to  believe  that  if  there  were 
any  such  central  masses,  they  would  be  not  only  larger  than 
the  other  stars,  but  brighter  in  a  yet  greater  proportion.  It 
is,  of  course,  possible  to  conceive  of  immense  dark  bodies, 
such  as  Lambert  supj)osed  to  exist,  but  we  cannot  but  believe 
the  existence  of  such  bodies  to  be  very  improbable.  Al- 
thougli  there  is,  as  we  have  seen,  great  diversity  among  the 
stars  in  respect  to  their  magni  udes,  there  are  none  of  them 
which  seem  to  have  that  commanding  preeminence  above 
their  fellows  which  the  sun  presents  above  the  planets  which 
surround  him. 

But  the  most  conclusi"^  proof  that  the  stars  do  not  revolve 
round  definite  attractin.g  centres  is  found  in  the  variety  and 
irregularity  of  their  proper  motions,  which  wc  have  already 
described.  We  have  shown  (1)  that  when  the  motions  of 
great  numbers  of  stars  are  averaged,  there  is  found  a  general 
preponderance  of  motions  from  the  constellation  Hercules, 
which  is  supposed  to  be  due  to  a  motion  of  our  sun  with  his 
attendant  planets  in  that  direction  ;  and  (2)  that  when  the 
motions  of  stars  in  the  same  region  are  compared,  there  is 
often  found  to  be  a  certain  resemblance  amoni»:  them.  But 
this  tendency  towards  a  regular  law  afPects  oidy  large  masses 
of  stars,  and  does  not  imj)ly  any  such  regularity  in  the  mo- 
tions of  individual  stars  as  would  be  apparent  if  they  moved 
in  regular  circular  orbits,  as  the  planets  move  round  the  sun. 
The  motion  of  each  individual  star  is  generally  so  entirely 
different  from  that  of  its  fellows  as  seemingly  to  preclude  all 
reasonable  probability  that  these  bodies  are  revolving  in  defi- 
nite orbits  around  great  centres  of  attraction. 

The  most  extraordinary  instances  of  the  irregularities  of 
which  we  speak  are  found  in  the  stars  of  unusually  rapid 
proper  motion,  which  are  moving  forward  at  such  a  rate  that 
the  gravitation  of  all  the  known  stars  cannot  stop  them  until 
they  shall  have  passed  through  and  beyond  the  visible  uni- 
verse. The  most  remarkable  of  tliese,  so  far  as  we  know,  is 
Groombridge  1830,  it  having  the  largest  apparent  proper  mo- 


486  THE  STELLAR   UNIVEESE. 

tion  of  any  known  star.  The  most  careful  determinations  of 
its  parallax  seem  to  show  that  its  distance  is  so  immense  that 
the  parallax  is  only  about  a  tenth  of  a  second ;  that  is,  a  line 
drawn  from  the  sun  to  the  earth  would  subtend  an  angle  of 
only  a  tenth  of  a  second  when  viewed  from  this  star.  But 
the  apparent  motion  of  the  star,  as  we  actually  see  it,  is  more 
than  seven  seconds  per  annum,  or  seventy  times  its  parallax. 
It  follows  that  the  star  moves  over  a  space  of  more  than  sev- 
enty times  the  distance  of  the  sun  from  us  in  the  space  of  a 
year.  If,  as  is  likely,  the  motion  of  the  star  is  oblicjue  to  the 
line  in  which  we  see  it,  its  actual  velocity  must  be  yet  greater. 
Leaving  this  out  of  account,  we  see  that  the  star  would  pass 
from  the  earth  to  the  sun  in  about  five  days,  so  that  its  veloci- 
ty probably  exceeds  two  hundred  miles  per  second. 

To  understand  what  this  enormous  velocity  may  imply,  we 
must  advert  to  the  theorem  of  gravitational  astronomy  that 
the  velocity  which  a  body  can  acquire  by  falling  towards  an 
attracting  centre  is,  at  each  point  of  its  path,  limited.  For  ex- 
ample, a  body  falling  from  an  infi'Mte  distance  to  the  earth's 
surface,  and  acted  on  by  the  attraction  of  the  earth  alone,  would 
acquire  a  velocity  of  only  about  seven  miles  per  second.  Vice 
ve)'sa,  a  body  projected  from  the  earth  with  tliis  velocity  would 
never  be  stopped  by  the  earth's  attraction  alone,  but  would 
describe  an  elliptic  orbit  round  the  sun.  If  the  velocity  ex- 
ceeded twenty-seven  miles  per  second,  the  attraction  of  the  sun 
himself  could  never  stop  it,  and  it  would  wander  forever 
through  the  stellar  spaces.  The  greater  the  distance  from  the 
sun  at  which  the  body  is  started,  the  less  the  velocity  which 
^ill  thus  carry  it  forever  away  from  the  snn.  At  the  o.'bit  of 
Uranus  the  required  velocity  would  be  only  six  miles  per  sec- 
ond ;  at  Neptune,  it  would  be  less  than  five  miles  per  second  ; 
half-way  between  the  sun  and  a  Centauri,  it  would  be  a  mile 
in  twelve  seconds,  or  a  fourth  the  speed  of  a  cannon-ball.  If 
we  knew  the  masses  of  each  of  the  stai-s,  and  their  arrange- 
ment in  space,  it  would  be  easy  to  compute  this  limiting  ve- 
locity for  a  body  falling  from  an  infinite  distance  to  any  point 
of  the  stellar  system.     If  the  motion  of  a  star  were  found  to 


IfO   TEE  STABS  REALLY  FORM  A   SYSTEMf  487 

exceed  this  limit,  it  would  show  that  the  star  did  not  belong 
to  the  visible  iniiverse  at  all,  but  was  only  a  visitor  %ing 
on  a  course  through  infinite  space  at  such  a  rate  that  the 
combined  attraction  of  all  the  stars  could  never  stop  it. 

Let  us  now  see  how  the  case  may  stand  with  onr  flying  star, 
and  what  relation  its  velocity  may  bear  to  the  probable  attrac- 
tion of  all  the  stars  which  exist  within  the  rancce  of  the  tel- 
escopo.  The  number  of  stars  actually  visible  with  the  most 
powerful  telescopes  probably  falls  short  of  fifty  millions ;  but, 
to  take  a  probable  outside  limit,  we  shall  suppose  that  within 
the  regions  occupied  by  the  farthest  stars  which  the  telescope 
will  show,  there  are  fifty  millions  more,  so  small  that  we  catmot 
see  them,  making  one  hundred  millions  in  all.  We  shall  also 
suppose  that  these  stars  have,  on  the  average,  five  times  the 
mass  of  the  sun,  and  that  they  are  spread  out  in  a  layer  across 
the  diameter  of  which  light  would  require  thirty  thousand  years 
to  pass.  Then,  a  mathematical  computation  of  the  attractive 
power  exerted  by  such  a  system  of  masses  shows  that  a  body 
falling  from  an  infinite  distance  to  the  centre  of  the  system 
would  acquire  a  velocity  of  twenty -five  miles  per  second. 
Vice  versa,  a  body  projected  from  the  centre  of  such  a  system 
with  a  velocity  of  more  than  twenty-five  miles  per  second  in 
any  direction  whatever  would  not  only  pass  entirely  through 
it,  but  would  fly  off  into  infinite  space,  never  to  return.  If  the 
body  were  anywhere  else  than  in  the  centre  of  the  system,  the 
velocity  necessary  to  carry  it  away  would  be  less  than  rlic 
limit  just  given.  But  this  calculated  limit  is  only  one  eighth 
the  probable  velocity  of  1830  Groombridge.  Tl;  force  re- 
quired to  impress  a  given  velocity  on  a  body  falling  through 
any  distance  is  proportional  to  the  square  of  the  velocity,  four 
times  the  force  being  required  to  give  double  the  velocity,  nine 
times  to  increase  it  threefold,  and  so  on.  To  give  eight  times 
the  velocity  would  require  sixty -four  times  the  attracting  mass. 
If,  then,  the  star  in  question  belongs  to  our  stellar  system,  the 
masses  or  extent  of  that  system  must  be  many  times  greater 
than  telescopic  observation  and  astronomical  resoarcih  indicate. 
We  may  place  the  dilemma  in  a  concise  form,  as  follows: 


<i8S  THE  STELLAR   UNIVERSE. 

Either  tlie  bodies  which  compose  our  universe  are  vastly 
more  massive  and  numerous  than  telescopic  examination 
seeins  to  indicate,  or  1830  Groombridge  is  a  runaway  star, 
Hying  on  a  boundless  course  through  infinite  space  with  such 
momentum  that  the  attraction  of  all  the  bodies  of  the  universe 
can  never  stop  it. 

Which  of  these  is  the  more  probable  alternative  we  cannot 
pretend  to  say.  That  the  star  can  neither  be  stopped,  nor  bent 
far  from  its  course  until  it  has  passed  the  extreme  limit  to 
which  the  telescope  has  ever  penetrated,  we  may  consider 
reasonably  certain.  To  do  this  will  require  two  or  three  mill- 
ions of  years.  Whether  it  will  then  be  acted,  on  by  attractive 
forces  of  which  science  has  no  knowledge,  and  thus  carried 
back  to  where  it  started,  or  whether  it  will  continue  straight 
forward  forever,  it  is  impossible  to  say. 

Much  the  same  dilemma  may  be  applied  to  the  past  history 
of  this  body.  If  the  velocity  of  two  hundred  miles  or  more 
per  second  with  which  it  is  moving  exceeds  an;y  that  could  be 
produced  by  the  attraction  of  all  the  other  bodies  in  the  uni- 
verse, then  it  must  have  been  flyiiig  forward  through  space 
from  the  beginning,  and,  having  come  from  an  infinite  dis- 
tance, must  be  now  passing  through  our  system  for  the  first 
and  only  time. 

It  may  be  asked  whether,  in  Lambert's  liypothesis  of  im- 
mense attracting  bodies,  invisible  on  account  of  their  being 
dark,  we  have  not  at  once  the  centres  required  to  give  general 
stability  to  the  stellar  system,  and  to  keep  tlie  star  of  which 
we  have  spoken  in  some  regular  orbit.  We  answer,  no.  To 
secure  such  stability,  stars  equally  distant  from  the  attracting 
centres  must  li^ove  with  nearly  the  same  velocity.  An  at- 
tracting centre  sufticiently  powerful  to  bring  a  body  moving 
two  hundred  miles  per  second  into  a  regular  orbit  would 
draw  most  of  the  other  stars  moving  with  small  velocities  into 
its  innnediate  neighborhood,  and  thus  subvert  the  system.  We 
thus  meet  the  double  difficulty  that  we  have  good  reason  to 
doubt  the  existence  of  these  opaque,  dark  bodies,  and  that  if 
they  did  exist,  they  would  not  fulfil  our  requirements. 


DO  THE  STARS  REALLY  FORM  A   SYSTEATf  489 

The  general  result  of  our  inquiry  is  that  the  stellar  uni- 
verse does  not  seem  to  possess  that  form  of  unvarying  stabil- 
ity which  we  see  in  the  solar  system,  and  that  the  stars  move 
in  irregular  courses  depending  on  their  situation  in  respect 
to  the  surrounding  stars,  and  probably  changing  as  this  situa- 
tion changes.  If  there  were  no  motion  at  all  among  the  stars, 
they  would  all  fall  to  a  common  centre,  and  universal  ruin 
would  be  the  result.  But  the  motions  which  we  actually  see 
are  sufficient  to  prevent  this  catastrophe,  by  supplying  each 
star  with  a  reserve  of  force  which  will  generally  keep  it  from 
actual  collision  with  its  neighbors.  If,  then,  any  one  star 
does  fall  towards  any  attracting  centre,  the  velocity  which  it 
acquires  by  this  fall  will  carry  it  away  again  in  some  other 
direction,  and  thus  it  may  keep  up  a  continuous  dance,  under 
the  influence  of  ever-varying  forces,  as  long  as  the  universe 
shall  exist  under  its  present  form. 

To  those  who  have  been  enraptured  with  the  sublime  specu- 
lations of  Kant  and  Lambert,  this  may  seem  an  unsatisfactory 
conclusion ;  while  to  those  who  look  upon  the  nuiterial  uni- 
verse as  something  made  to  last  forever,  it  may  seem  inq)roba- 
ble.  But  when  we  consider  the  immense  periods  which  would 
be  required  for  the  mutual  gravitation  of  the  stars  to  effect 
any  great  change  in  the  stellar  system,  we  may  be  led  to  alter 
such  views  as  these.  We  have  shown  that  tens  of  thousands 
of  years  would  be  required  to  make  any  great  change  in  the 
arrangement  of  the  stars  which  we  see  with  the  naked  eye. 
The  time  required  for  all  the  stars  visible  with  the  telescope 
to  fall  together  by  their  own  attraction  is  to  be  counted  by 
millions  of  years.  If  the  universe  had  existed  in  its  present 
state  from  eternity,  and  were  to  exist  forever,  the  immensity 
of  these  periods^  would  not  be  at  all  to  the  point,  because  a 
million  of  years  is  no  more  a  part  of  eternity  than  a  single 
day.  But  all  modern  science  seems  to  point  to  the  flnite 
duration  of  our  system  in  its  present  form,  and  to  carry  us 
back  to  the  time  when  neither  sun  nor  planet  existed,  save  as 
a  mass  of  glowing  gas.  How  far  back  that  was,  it  cannot  tell 
us  with  certainty;  it  can  only  say  that  the  period  is  counted 


490  THE  STELLAR   UNIVERSE. 

by  millions  of  years,  but  probably  not  by  hundreds  of  mill- 
ions. It  also  points  forward  to  the  time  when  the  sun  and 
stars  shall  fade  away,  and  nature  shall  be  enshrouded  in  djirk- 
ness  and  death,  unless  some  power  now  unseen  shall  uphold 
or  restore  her.  The  time  required  for  this  catastrophe  cannot 
be  calculated  •  but  it  is  probably  not  so  great  that  the  stellar 
system  can,  in  the  mean  time,  be  subverted  by  the  mutual 
gravitation  of  its  members. 

It  would  thus  appear  as  if  those  nicely  arranged  adjust- 
ments which  secure  stability  and  uniformity  of  motion  are 
not  found  where  tlie}'  are  not  necessary  to  secure  the  system 
from  subversion  during  the  time  it  is  to  last,  much  as  the 
wheel  of  an  engine  which  is  to  make  but  two  or  three  revo- 
lutions wliile  the  engine  endures  need  not  be  adjusted  to 
make  thousands  of  revolutions.  Tlie  bodies  which  form  our 
solar  system  are,  on  the  other  hand,  like  wheels  which  have 
to  make  millions  of  revolutions  before  they  stop.  Unless  there 
is  a  constant  balance  between  the  opposing  forces  under  the 
influence  of  which  they  move,  there  must  be  a  disarrangement 
of  the  movement  long  before  the  engine  wears  out.  Thus, 
although  the  present  arrangement  of  the  stars  may  be  studied 
without  any  reference  to  their  origin,  yet,  when  we  seek  to 
penetrate  the  laws  of  their  motion,  and  foresee  the  changes 
of  state  to  which  their  motions  may  give  rise,  we  are  brought 
to  face  the  question  of  their  duration,  and  hence  of  their  be- 
ginning and  end. 


TEE  COSMOGONY.  491 


CHAPTER  III. 

THE     COSMOGONY. 

The  idea  tliat  the  world  has  not  endured  forever  in  the 
form  in  which  we  now  see  it,  but  that  there  was  a  time  when 
it  either  did  not  exist  at  all,  or  existed  only  as  a  mass  "with- 
out form,  and  void,"  is  one  which  we  iind  to  have  been  always 
held  by  mankind.  The  "  chaos  "  of  the  Greeks — the  rude  and 
formless  materials,  subject  to  no  law,  out  of  which  all  things 
were  formed  by  the  creative  power — corresponds  in  a  striking 
manner  to  the  nebulous  masses  of  modern  astronomy.  These 
old  ideas  of  chaos  were  expressed  by  Milton  in  the  second 
book  of  "Paradise  Lost,"  before  such  a  thing  as  a  nebula 
could  be  said  to  be  known,  and  he  would  be  a  bold  astrono- 
mer who,  in  giving  a  description  of  the  primeval  nebulous 
mass,  would  attempt  to  improve  on  the  great  poet : 

"a  dark, 


Illimitable  ocean,  without  bound, 

Without  dimension,  where  length,  breadth,  and  height. 

And  time  and  place,  are  lost ;  where  eldest  Night 

And  Chaos,  ancestors  of  Nature,  hold 

Eternal  anarchy  amidst  the  noise 

Of  endless  wars,  and  by  confusion  stand  : 

For  hot,  cold,  moist,  and  dry,  four  champions  fierce, 

Strive  here  for  mastery,  and  to  battle  bring 

Their  embryon  atoms. 

«  iK  in  >tc  *  it<  « 

Chaos  umpire  sits. 
And  by  decision  more  embroils  the  fray 
By  which  he  reigns  :  next  him,  high  arbiter, 
Chance  governs  all.     Into  this  wild  abyss 
The  womb  of  Nature,  and  perhaps  her  grave. 
Of  neither  sea,  nor  shore,  nor  air,  nor  fire, 
But  all  these  in  their  pregnant  causes  mixed 
Confusedly,  and  which  thus  must  ever  fight, 


492  THE  STELLAR   UNIVERSE. 

Unless  the  aliniglity  Maker  them  ordain 
His  dark  materials  to  create  more  worlds — 

*  *  it<  «  >tc  itc 

Some  tumultuous  cloud 
Instinct  with  fire  and  nitre," 

If  WG  classify  men's  ideas  of  the  cosmogoiiy  according  to 
the  data  on  which  they  are  founded,  we  shall  lind  them  divis- 
ible into  three  classes.  The  first  class  comprises  those  formed 
before  the  discovery  of  the  theory  of  gravitation,  and  which, 
for  this  reason,  however  correct  they  might  have  been,  had  no 
really  scientific  foundation.  The  second  are  those  founded  on 
the  doctrine  of  gravitation,  but  without  a  knowledge  of  the 
modern  theory  of  the  conservation  of  force ;  while  the  third 
are  founded  on  this  theory.  It  must  not  be  supposed,  how- 
ever, that  the  ideas  of  the  last-mentioned  class  are  antagonistic 
to  those  of  the  other  classes.  Kant  and  Laplace  founded  the 
nebular  hypothesis  on  the  theor}'  of  gravitation  alone,  the  con- 
servation of  force  being  then  entirely  nnknown.  It  was,  there- 
fore, incomplete  as  it  came  from  their  hands,  but  not  neces- 
sarily erroneous  in  its  fundamental  conceptions. 

The  consideration  of  the  ancient  ideas  of  the  origin  of  the 
world  belongs  rather  to  the  history  of  philosophy  than  to  as- 
tronomy, for  the  reason  that  they  were  of  necessity  purely 
speculative,  and  reflected  rather  the  mode  of  thought  of  the 
minds  in  which  they  originated  than  any  definite  system  of 
investigating  the  operations  of  nature.  The  Hindoo  concep- 
tion of  Brahma  sitting  in  meditation  on  a  lotus-leaf  through 
long  ages,  and  then  producing  a  golden  e^g  as  large  as  the 
miiverse,  out  of  which  the  latter  was  slowly  evolved,  is  not 
founded  on  even  the  crudest  observation,  but  is  purely  a  result 
of  the  speculative  tendency  of  the  Hindoo  mind.  Tiie  Jew- 
ish cosmogony  is  the  expression  of  the  monotheistic  views  of 
that  people,  and  of  the  identity  of  their  tutelary  divinity  with 
the  maker  of  heaven  and  earth.  Ilipparchus  and  Ptolemy 
showed  the  scientific  turn  of  their  minds  by  confining  them- 
selves to  the  examination  of  the  universe  as  it  is,  without  mak- 
ing any  vain  effort  to  trace  its  origin. 


THE  MODERN  NEBULAR  HYPOTHESIS.  493 

Though  tlie  systems  to  which  we  refer  are  essentially  un- 
scientific, it  must  not  be  supposed  that  they  were  all  errone- 
ous in  their  results,  or  that  they  belong  exclusively  to  ancient 
times.  Thus,  the  views  of  Swedenhorg,  though  they  belong 
to  the  class  in  question,  are  remarkably  in  accordance  with 
recent  views  of  the  subject  as  regards  the  actual  changes  which 
took  place  during  the  formation  of  the  planets.  A  great  deal 
of  what  is  written  on  the  subject  at  present  is  to  be  included 
in  this  same  ancient  class,  as  being  the  production  of  men  who 
are  not  mathematicians  or  working  astronomers,  and  who, 
therefore,  cannot  judge  whether  their  views  are  in  accordance 
with  mechanical  laws  and  with  the  facts  of  observation.  Pass- 
ing over  all  speculation  of  this  sort,  no  matter  when  or  by 
whom  produced,  we  shall  consider  in  historical  order  the  works 
of  those  who  have  actually  contributed  to  placing  the  laws  of 
cosmogony  on  a  scientific  foundation. 

§  1.  The  Modern  Nebular  Hypothesis. 

From  a  purely  scientific  point  of  view,  Kant  has  probably 
the  best  risrht  to  be  regarded  as  the  founder  of  the  nebular 
hypothesis,  because  he  based  it  on  an  examination  of  the  actual 
features  of  the  solar  system,  and  on  the  Newtonian  doctrine 
of  the  mutual  gravitation  of  all  matter.  His  reasoning  is 
briefly  this:  Examining  the  solar  system,  we  find  two  remark- 
able features  presented  to  our  consideration.  One  is  that  six 
planets  and  nine  satellites  (the  entire  number  then  known) 
move  around  the  sun  in  circles,  not  only  in  the  same  direction 
in  which  the  sun  himself  revolves  on  his  axi^,  but  very  nearly 
in  the  same  plane.  This  common  feature  of  the  motion  of 
so  many  bodies  could  not,  by  any  reasonable  possibility,  have 
been  a  result  of  chance;  we  are,  therefore,  forced  to  believe 
that  it  must  be  the  result  of  some  common  cause  originally 
acting  on  all  the  planets. 

On  the  other  hand,  when  we  consider  the  spaces  in  which 
the  planets  move,  we  find  them  entirely  void,  or  as  good  as 
void;  for  if  there  is  any  matter  in  them, it  is  so  rare  as  to  be 
without  effect  on  the  planetary  motions.     There  is,  therefore. 


494  THE  STELLAR   UNIVERSE. 

no  material  connection  now  existing  between  the  planets 
through  whicih  they  might  hav"  been  forced  to  take  up  a  com- 
mon direction  of  motion.  How,  then,  are  we  to  reconcile  this 
common  motion  with  the  absence  of  all  material  connection  ? 
The  most  natural  way  is  to  suppose  that  tliere  was  once  some 
such  connection  which  brouglit  about  tlie  uniformity  of  mo- 
tion which  we  observe  ;  that  the  materials  of  which  the  plan- 
ets are  formed  once  tilled  the  whole  space  between  them.  "  I 
assume,"  says  Kant, "  that  all  the  materials  out  of  which  the 
bodies  of  our  solar  system  were  formed  were,  in  the  begin- 
ning of  things,  resolved  in  their  original  elements,  and  filled  all 
the  space  of  the  universe  in  which  these  bodies  now  move." 
There  was  no  formation  in  this  chaos,  the  formation  of  sepa- 
rate bodies  by  the  mutual  gravitation  of  parts  of  the  mass  be- 
ing a  later  occurrence.  But,  naturally,  some  parts  of  the  mass 
would  be  more  dense  than  others,  and  would  thus  gather 
around  them  the  rare  matter  which  filled  the  intervening: 
spaces.  The  larger  collections  thus  formed  would  draw  the 
smaller  ones  into  them,  and  this  process  would  continue  until 
a  few  round  bodies  had  taken  the  place  of  the  original  chaotic 
mass. 

If  we  examine  the  residt  of  this  hypothesis  by  the  light  of 
modern  science,  we  shall  readily  see  that  all  the  bodies  thus 
formed  would  be  drawn  to  a  common  centre,  and  thus  we 
should  have,  not  a  collection  of  bodies  like  the  solar  system, 
but  a  single  sun  formed  by  the  combination  of  them  all.  In 
attempting  to  show  how  the  smaller  masses  would  be  led  to 
circulate  around  the  larger  ones  in  circular  orbits,  Kant's  rea- 
soning ceases  to  be  satisfactory.  He  seems  to  think  that  the 
motion  of  rotation  could  be  produced  indirectly  by  the  repul- 
sive forces  acting  among  the  rarer  masses  of  the  condensing 
matter,  which  would  give  rise  to  a  whirling  motion.  But  the 
laws  of  mechanics  show  that  the  sum  total  of  rotary  motion  in 
a  system  can  never  be  increased  or  diminished  by  the  mutual 
action  of  its  separate  parts,  so  that  the  present  rotary  motions 
of  the  sun  and  planets  must  be  the  equivalent  of  that  which 
they  had  from  the  beginning. 


the' MODERN  NEBULAR  HYPOTHESIS.  495 

HerscheVs  Hypothesis.  —  It  Is  remarkable  that  the  idea  of 
the  gradual  transimitation  of  nebulaj  into  stars  seems  to  have 
been  suirgested  to  Ilerschel,  not  by  the  relations  of  the  solar 
system,  but  by  his  examinations  of  the  nebulie  themselves. 
Many  of  these  bodies  seemed  to  him  to  be  eomposed  of  im- 
mense masses  of  phosphorescent  vapor,  and  he  conceived  that 
these  masses  must  be  gradually  condensing,  each  around  its 
own  centre,  or  around  those  parts  where  it  is  most  dense,  until 
it  should  be  transmuted  into  a  star  or  a  cluster  of  stars.  On 
classifying  the  numerous  nebulae  which  he  discovered,  it 
seemed  to  him  that  he  could  see  each  stage  of  this  operation 
going  on  before  his  eyes.  There  were  the  large,  faint,  diffused 
nebulae,  in  which  the  process  of  condensation  seemed  to  have 
hardly  begun ;  the  smaller  but  brighter  ones,  which  had  been 
so  far  condensed  that  the  central  parts  would  soon  begin  to 
form  into  stars ;  yet  others,  in  which  stars  had  actually  begun 
to  form ;  and,  finally,  star  clusters  in  which  the  condensation 
was  complete.  As  Laplace  observes,  Ilerschel  followed  the 
condensation  of  the  nebulae  in  much  the  same  way  that  we 
can,  in  a  forest,  study  the  growth  of  the  trees  by  comparing 
those  of  the  different  ages  which  the  forest  contains  at  the 
same  time.  The  spectroscopic  revelations  of  the  gaseous  nat- 
ure of  the  true  nebulae  tend  to  strengthen  these  views  of  Iler- 
schel, and  to  con^rm  us  in  the  opinion  that  these  masses  will 
all  at  some  time  condense  into  stai-s  or  clusters  of  stars. 

Laplace  s  View  of  the  Nebular  II//pothcsis. — Laplace  was  led 
to  the  nebular  hypothesis  by  considerations  very  similar  to 
those  presented  by  Kant  a  few  years  before.  The  remarkable 
uniformity  among  the  directions  of  rotation  of  the  planets  be- 
ing something  which  could  not  have  been  the  result  of  chance, 
he  sought  to  investigate  its  probable  cause.  This  cause,  he 
thought,  could  be  nothing  else  than  the  atmosphere  of  the  sun, 
which  once  extended  so  far  out  as  to  fill  all  the  space  now  oc- 
cupied by  the  planets.  He  does  not,  like  Kant,  begin  with  a 
chaos,  out  of  which  order  was  slowly  evolved  by  the  play  of 
attractive  and  repulsive  forces,  but  with  the  sun,  surrounded 
by  this  immense  fiery  atmosphere.     Knowing,  from  mechan- 


496  THE  STELLAR   UNIVERSE. 

ical  laws,  that  the  sum  total  of  rotary  motion  now  seen  in  the 
planetary  system  must  have  been  there  from  the  beginning,  he 
conceives  the  immense  vaporous  mass  forming  the  sun  and 
his  atmosphere  to  have  had  a  slow  rotation  on  its  axis.  The 
mass  being  intensely  hot  would  slowly  cool  off,  and  as  it  did  so 
would  contract  towards  the  centre.  As  it  contracted,  its  ve- 
locity of  rotation  would,  in  obedience  to  one  of  the  funda- 
mental laws  of  mechanics,  constantly  increase,  so  that  a  time 
would  arrive  when,  at  the  outer  boundary  of  the  mass,  the  cen- 
trifugal force  due  to  the  rotation  would  counterbalance  the  at- 
tractive force  of  the  central  mass.  Then,  those  outer  portions 
woidd  be  left  behind  as  a  revolving  ring,  while  the  next  inner 
portions  would  continue  to  contract  until,  at  their  boundary, 
the  centrifugal  and  attractive  forces  would  be  again  balanced, 
when  a  second  ring  would  be  left  behind,  and  so  on.  Thus, 
instead  of  a  continuous  atmosphere,  the  sun  would  be  sur- 
rounded by  a  series  of  concentric  revolving  rings  of  vapor. 

]S"ow,  how  would  these  rings  of  vapor  behave  ?  As  they 
cooled  off,  their  denser  materials  would  condense  first,  and 
thus  the  ring  would  be  composed  of  a  mixed  mass,  partly  solid 
and  partly  vaporous  the  quantity  of  solid  matter  constantly 
in(;reasing,  and  that  of  vapor  diminishing.  If  the  ring  were 
perfectly  uniform,  this  condensing  process  M'^ould  take  place 
equally  all  around  it,  and  the  ring  would  thus  be  broken  up 
into  a  group  of  small  planets,  like  that  which  we  see  between 
Mars  and  Jupiter.  But  we  should  expect  that  in  general 
some  portions  of  the  ring  would  be  much  denser  than  others, 
and  the  denser  portions  would  gradually  attract  the  rarer  por- 
tions around  it  until,  instead  of  a  ring,  we  should  have  a  sin- 
gle mass,  composed  of  a  nearly  solid  centre  surrounded  by  an 
immense  atmosphere  of  fiery  vapor.  This  condensation  of  the 
ring  of  vapor  around  a  single  point  would  have  produced  no 
change  in  the  amount  of  rotary  motion  originally  existing  in 
the  ring  ;  the  planet,  surrounded  by  its  fiery  atmosphere,  would 
therefore  be  in  rotation,  and  would  be,  in  miniature,  a  repro- 
duction of  the  case  of  the  sun  surrounded  by  his  atmosphere 
with  which  we  set  out.     In  the  same  way  that  the  solar  at- 


THE  MODERN  NEBULA  11    UYI'OTIIESIS.  407 

mospliere  formed  itself  first  into  rings,  and  then  tliese  rings 
condensed  into  i)lanets,  so,  if  the  ])liinetary  atmosplieres  were 
sufficiently  extensive,  they  would  form  themselves  into  rings, 
and  these  rings  would  condense  into  satellites.  In  the  case  of 
Saturn,  however,  one  of  the  rings  was  so  perfectly  uniform 
that  there  could  be  no  denser  portion  to  draw  the  rest  of 
the  ring  around  it,  and  thus  we  have  the  well-known  rings 
of  Saturn. 

If,  among  the  materials  of  the  solar  atmosphere,  there  were 
any  so  rare  and  volatile  that  they  would  not  unite  themselves 
either  into  a  ring  or  around  a  j)lanet,  they  would  continue  to 
revolve  around  the  sun,  presenting  an  ap])earancc  like  that 
of  the  zodiacal  light.  They  would  offer  no  appreciable  re- 
sistance to  the  motion  of  the  i)lanets,  not  only  on  account  of 
their  extreme  rarity,  but  because  their  motion  would  be  the 
same  as  that  of  the  planets  which  move  among  them. 

Such  is  the  celebrated  nebular  hypothesis  of  Laplace  which 
has  given  rise  to  so  much  discnssion.  It  commences,  not  with 
a  purely  nebulous  mass,  but  with  the  sun  surrounded  by  a 
liery  atmosphere,  out  of  which  the  planets  were  formed.  On 
this  theory  the  sun  is  older  than  the  planets;  otherwise  it 
would  have  been  impossible  to  account  for  the  slow  rotation 
of  the  sun  npon  his  axis.  If  Ins  body  had  been  formed  of  ho- 
mogeneous matter  extending  out  uniformly  to  near  the  orbit 
of  Mercury,  it  would  not  have  condensed  into  a  globe  revolv- 
ing on  its  axis  in  twenty-five  days,  but  into  a  fiat,  almost  lens- 
shaped,  body,  which  would  have  been  kept  from  forming  a 
sphere  by  the  centrifugal  force.  But  the  denser  materials  be- 
ing condensed  first,  perhaps  into  such  a  body  as  we  described, 
the  friction  of  the  uncondensed  atmosjihere  would  have  di- 
minished the  rotation  of  the  sun,  the  rotating  energy  which  he 
lost  being  communicated  to  the  embryo  planets  and  throwing 
them  farther  away. 

In  accordance  with  the  hypothesis  of  Laplace,  it  has  al- 
ways been  supposed  that  the  outer  planets  were  formed  first. 
There  is,  however,  a  weak  point  in  Laplace's  theory  of  the  for- 
mation of  rings.     He  supposed  that  when  the  centrifugal  and 

33 


498  THE  STELLAR   UNIVERSE. 

centripetal  forees  balanced  each  other  at  the  outer  limit  of 
the  revolving  mass,  the  outer  portions  were  separated  from  the 
rest,  which  continued  to  drop  towards  the  centre.  If  the  plan- 
etary rings  were  formed  in  this  way,  then,  after  each  ring  was 
thrown  off,  the  atmosphere  must  have  condensed  to  nearly 
half  its  diameter  before  another  would  have  been  thrown  off, 
because  we  see  that  each  planet  is,  on  the  whole,  nearly  twice 
as  far  as  the  one  next  within  it.  But  there  being  no  cohe- 
sion between  particles  of  vapor,  such  thro  wing-off  of  immense 
masses  of  the  outside  portions  of  the  revolving  mass  was  im- 
possible. The  moment  the  forces  balanced,  the  outer  portions 
of  the  mass  would,  indeed,  cease  to  drop  towards  the  sun,  and 
would  partially  separate  from  the  portions  next  to  it ;  then 
these  would  separate  next,  and  so  on  ;  that  is,  there  would  be 
a  constant  dropping-off  of  matter  from  the  outer  portions,  so 
that,  instead  of  a  series  of  rings,  there  would  have  been  a  flat 
disk  formed  of  an  infinite  number  of  concentrating  rings  all 
joined  together. 

If  we  examine  the  subject  more  closely,  we  shall  see  that 
the  whole  reasoning  by  which  it  is  supposed  that  the  inner 
portions  of  the  mass  would  drop  away  from  the  outer  ones 
needs  important  modifications.  In  its  primeval  state,  when  it 
extended  far  beyond  the  present  confines  of  the  solar  system, 
the  rare  nebulous  atmosphere  must  have  been  nearly  spherical. 
As  it  gradually  contracted,  and  the  effect  of  centrifugal  force 
thus  became  more  marked,  it  would  have  assumed  the  form 
of  an  oblate  spheroid.  When  the  contraction  had  gone  so 
far  that  the  centrifugal  and  attracting  forces  nearly  balanced 
each  other  at  the  outer  equatorial  limit  of  the  mass,  the  result 
would  have  been  that  contraction  in  the  direction  of  the  e(j^ua- 
tor  would  cease  entirely,  and  be  confined  to  tlie  polar  regions, 
each  particle  dropping,  not  towards  the  sun,  but  towards  the 
plane  of  the  solar  equator.  Thus,  we  should  have  a  constant 
flattening  of  the  spheroidal  atmosphere  until  it  was  reduced 
to  a  thin  flat  disk.  Tliis  disk  might  then  separate  itself  into 
rings,  which  would  form  planets  in  much  the  same  way  that 
Laplace  supposed.     But  there  would  probably  be  no  marked 


VROGliESSIVE  CHANGES  IN  OUR  SYSTEM.  499 

difference  in  the  age  of  the  planets ;  quite  likely  the  smaller 
inner  rings  would  condense  into  planets  more  rapidly  than  the 
wide-spread  outer  ones. 

Kant  and  Laplace  may  be  said  to  have  arrived  at  the  neb- 
nlar  hypothesis  by  reasoning  forward,  and  showing  how,  by 
supposing  that  the  space  now  occupied  by  the  solar  system 
was  once  filled  by  a  chaotic  or  vaporous  mass,  from  which  the 
planets  were  formed,  the  features  presented  by  this  system 
could  be  accounted  for.  Wc  are  now  to  show  how  our  mod- 
ern science  reaches  a  similar  result  by  reasoning  backward 
from  actions  which  we  see  going  on  before  our  eyes. 

§  2.  Progressive  Cliaivjes  in  our  System. 

During  the  short  period  within  which  accurate  observations 
have  been  made,  no  actual  permanent  change  has  been  ob- 
served in  our  system.  The  earth,  sun,  and  i)lanets  remain  of 
the  same  magnitude,  and  present  the  same  appearance  as  al- 
ways. The  stars  retain  their  brilliancy,  and,  for  the  most  part, 
the  nebulae  their  form.  ISot  the  slightest  variation  has  been 
detected  in  the  amount  of  heat  received  from  the  sun,  or  in 
the  average  number  and  extent  of  the  spots  on  his  surface. 
And  yet  we  have  reason  to  believe  that  these  things  are  all 
changing,  and  that  the  time  will  come  when  the  state  of  the 
universe  will  be  very  different  from  that  in  which  we  now  see 
it.  How  a  change  may  be  inferred  when  none  is  actually  vis- 
ible may  be  shown  by  a  simple  example. 

Suppose  an  inquiring  person,  walking  in  what  he  sup- 
posed to  be  a  deserted  building,  to  find  a  clock  running.  If 
he  is  ignorant  of  mechanics,  he  will  see  no  reason  why  it  may 
not  have  been  running  just  as  he  now  sees  it  for  an  indefinite 
period,  and  why  the  pendulum  may  not  continue  to  vibrate, 
and  the  hands  to  go  through  their  revolutions,  so  long  as  the 
fabric  shall  stand.  lie  sees  a  continuous  cycle  <»f  motions,  and 
can  give  no  reason  why  they  should  not  have  beer  going  on 
since  the  clock  was  erected,  and  continue  to  go  on  till  it  shall 
deca3^  But  let  him  be  instructed  in  the  laws  of  mechanics, 
and  let  him  inquire  into  the  force  which  keeps  the  hands  and 


500  THE  STELLAR   UNIVERSE. 

pendulum  in  motion,  lie  will  then  find  that  this  force  is 
transmitted  to  the  penduliun  throuii;h  a  train  of  wheels,  each 
of  which  moves  many  times  slower  than  that  in  front  of  it, 
and  that  the  lirst  wheel  is  acted  u[)on  by  a  weight,  with  which 
it  is  connected  by  a  cord,  lie  can  sec  a  slow  motion  in  the 
wheel  which  acts  on  the  pendulum,  and  perhaps  in  the  one 
next  behind  it,  while  during  the  short  time  ho  has  for  exami- 
nation ho  can  see  no  motion  in  the  others.  I>ut  if  he  sees  how 
the  wheels  act  on  each  other,  he  will  know  tiiat  they  nmst  all 
be  in  motion ;  and  when  iie  traces  the  motion  back  to  the  first 
wheel,  he  sees  that  its  motion  must  be  ke{)t  U])  by  a  gradual 
falling  of  the  weight,  though  it  seems  to  remain  in  the  same 
position,  lie  can  then  say  "with  entire  certainty:  "1  do  not  sec 
this  weight  move,  but  1  know  it  nmst  be  gradually  approach- 
ing the  bottom,  because  1  see  a  system  of  moving  machinery, 
the  progress  of  which  necessarily  involves  such  a  slow  falling 
of  the  weiijcht.  Knowinej  the  number  of  teeth  in  each  wheel 
and  pinion,  I  can  compute  how  many  inches  it  falls  each  day; 
and  seeing  how  much  room  it  has  to  fall  in,  I  can  tell  how 
many  days  it  will  take  to  reach  the  bottom.  When  this  is 
done,  I  see  that  the  clock  must  stop,  because  it  is  only  the  fall- 
ing of  the  weight  that  keeps  its  pendulum  in  motion.  More- 
over, 1  see  that  the  weight  nmst  have  been  higlier  yesterday 
than  it  is  to-day,  and  yet  higher  the  day  before,  so  that  1  can 
calculate  its  position  backward  as  well  as  forward.  By  this 
calculation  I  see  backward  to  a  time  when  the  weight  was 
at  the  top  of  its  course,  higher  than  which  it  could  not  be. 
Thus,  although  I  see  no  motion,  I  see  with  the  eye  of  reason 
that  the  weiijht  is  running  throuo^h  a  certain  course  from  the 
top  of  the  clock  to  the  bottom ;  that  some  power  must  have 
wound  it  up  and  started  it;  and  that  unless  the  same  power 
intervenes  again,  the  weight  nmst  reach  the  bot+^om  in  a  cer- 
tain number  of  days,  and  the  clock  must  then  stop." 

The  corresponding  progressive  change  exhibited  by  the 
operations  of  nature  consists  in  a  constant  transformation  of 
motion  into  heat,  and  the  constant  loss  of  that  heat  bv  radia- 
tion  into  si)ace.     As  Sir  William  Thomson  lias  expressed  it, 


rmHUiESSlVE  CHANGES  IN  OVli  SYSTEM.  501 

a  constant  "dissipation  of  energy"  is  going  on  in  nature. 
We  all  know  that  tlio  sun  lias  beoji  radiating  heat  into  si)aee 
dnring  the  whole  course  of  his  existence.  A  bniali  portion  of 
this  heat  strikes  the  earth,  and  supports  life  and  motion  on  its 
snrfaco.  All  this  portion  of  the  sun's  heat,  after  performing 
its  function,  is  radiated  oft'  into  space  by  the  earth  itself.  The 
portion  of  the  sun's  radiant  heat  received  hy  the  earth  is,  how- 
ever, comparatively  insignificant,  since  om-  luminary  radiates 
in  every  direction  cnpuilly,  while  the  earth  can  receive  oidy  a 
part  represented  hy  the  ratio  which  its  apparent  angular  mag- 
nitude as  seen  from  the  sun  hoars  to  the  wliole  celestial  sphere, 
which  a  simple  calculation  shows  to  be  the  ratio  of  1  to 
2,170,000,000.  The  stars  radiate  heat  as  well  as  the  sun. 
The  heat  received  from  them,  when  condensed  in  the  focus  of 
a  telescope,  has  been  rendered  sensible  by  the  thermo-imdti- 
plier,  and  there  is  every  reason  to  believe  that  stellar  heat  and 
light  bear  the  same  ])roportion  to  each  other  that  solar  heat 
and  light  do.  Wherever  there  is  white  stellar  light,  there 
must  be  stellar  heat ;  and  as  we  have  found  tluit  the  stars  in 
general  give  more  light  than  the  sun,  we  have  reason  to  be- 
lieve that  they  give  more  heat  also.  Thus  we  have  a  contin- 
uous radiation  from  all  the  visible  bodies  of  the  nniverse, 
which  nuist  have  been  going  on  from  the  beginning. 

Until  ({uite  rec-ently,  it  was  not  known  that  this  radiation 
involved  the  expenditure  of  a  something  necessarily  limited  in 
supply,  and,  consequently,  it  was  not  known  but  that  it  might 
continue  forever  without  any  loss  of  power  on  the  part  of  the 
sun  and  stars.  IJut  it  is  now  known  that  heat  cannot  be  pro- 
duced except  by  the  expenditure  of  force,  actual  or  potential, 
in  some  of  its  forms,  and  it  is  also  known  that  the  available 
supply  of  force  is  necessarily  limited.  (Jne  of  the  best-estab- 
lished doctrines  of  modern  science  is  that  force  can  no  more 
be  })roduced  from  nothing  than  matter  can :  to  find  it  so  pro- 
duced would  be  as  com})lete  a  miracle  as  to  see  a  globe  created 
from  nothing  before  our  eyes.  Hence,  this  radiation  cannot 
go  on  forever  unless  the  force  expended  in  producing  the  heat 
be  returned  lo  the  sun  in  some  form.     That  it  is  not  now 


502  THE  STELLAR   UNIVERSE. 

SO  returned  we  may  regard  as  morally  certain.  There  is  no 
known  law  of  radiation,  except  that  it  proceeds  out  in  straight 
lines  from  the  radiating  centre.  If  the  heat  were  returned 
back  to  the  sun  from  space,  't  would  have  to  return  to  the 
centre  from  all  directions ;  the  earth  would  then  intercept  as 
much  of  the  incoming  as  of  the  outgoing  heat ;  that  is,  we 
should  receive  as  much  heat  from  the  sky  at  night  as  from 
the  sun  by  day.  We  know  very  well  that  this  is  not  the  case ; 
indeed,  there  is  no  evidence  of  any  heat  at  all  reaching  us  from 
space  except  what  is  radiated  from  the  stars. 

Since,  then,  the  solar  heat  does  not  now  return  to  the  sun, 
we  have  to  inquire  what  becomes  of  it,  and  whether  a  com- 
pensation may  not  at  some  time  be  effected  whereby  all  the 
lost  heat  will  be  received  back  again.  Now,  if  we  trace  the 
radiated  heat  into  the  wilds  of  space,  we  may  make  three  pos- 
sible hypotheses  respecting  its  ultimate  destiny : 

1.  We  may  suppose  it  to  be  absolutely  annihilated,  just  as  it 
was  formerly  supposed  to  be  annihilated  when  it  was  lost  by 
friction. 

2.  It  may  continue  its  onward  course  through  space  forever. 

3.  It  may,  through  some  agency  of  which  we  have  no  con- 
ception, be  ultimately  gathered  and  returned  to  the  sources 
from  which  it  emanated. 

The  fii*st  of  these  hypotheses  is  one  which  the  scientific 
thinkers  of  the  present  day  would  not  regard  as  at  all  philo- 
sophical. In  our  scientific  philosophy,  the  doctrine  that  force 
cannot  be  annihilated  is  coequal  with  that  that  it  cannot  be 
created ;  and  the  inductive  processes  on  which  the  latter  doc- 
trine is  founded  are  almost  as  unimpeachable  as  those  from 
which  we  con(!lude  that  matter  cannot  be  created.  At  the 
same  time,  it  might  be  maintained  that  all  these  doctrines  re- 
specting the  uncreatableness  and  indestructibility  of  matter 
and  force  can  have  no  proper  foundation  except  induction 
from  experiment,  and  that  the  absolute  truth  of  a  doctrine 
like  this  cannot  be  proved  by  induction.  Especially  may  this 
be  claimed  in  respect  of  force.  The  most  careful  measures  of 
force  which  we  can  make  under  all  circumstances  show  that  it 


PltOGRESSIVE  CHANGES  IN  OUR  SYSTEM.  503 

is  subject  to  no  sensible  loss  by  either  transmission  or  transfor- 
mation. But  this  alone  does  not  prove  that  it  can  be  subject 
to  no  loss  in  a  passage  through  space  requiring  hundreds  of 
thousands  or  millions  of  years.  There  is  also  this  essential 
difference  between  force  and  matter,  that  we  conceive  the  lat- 
ter as  made  up  of  individual  paits  which  preserve  their  iden- 
tity through  all  the  changes  of  form  which  they  undergo; 
while  force  is  something  in  which  we  do  not  conceive  of  any 
such  identity.  Thus,  when  I  allow  a  drop  of  water  to  evapo- 
rate from  my  hand,  I  can  in  imagination  trace  each  molecule 
of  water  through  the  air,  into  the  clouds,  and  down  to  the 
earth  again  in  f^ome  particular  drop  of  rain,  so  that,  if  I  only 
had  the  means  of  actually  tracing  it,  I  could  say,  "  This  cup 
contains  one,  or  two,  or  twenty  of  the  identical  molecules 
which  evaporated  from  my  hand  a  week  or  a  month  ago." 
It  is  on  this  idea  of  the  separate  identity  of  each  molecule 
of  matter  that  our  opinion  of  the  indestructibility  of  matter  is 
founded,  because  matter  cannot  be  destroyed  without  destroy- 
ing individual  molecules,  and  any  cause  which  could  destroy  a 
single  molecule  might  equally  destroy  all  the  molecules  in  the 
universe. 

But  neither  parts  nor  identity  is  possible  in  force.  A  cer- 
tain amount  of  heat  may  l)e  expended  in  simply  raising  a 
weight.  Here  heat  has  disappeared,  and  is  replaced  by  a 
mere  change  of  position  —  something  which  cannot  be  con- 
ceived as  identical  with  it.  If  we  let  the  weight  drop,  the 
same  amount  of  heat  will  be  reproduced  that  was  expended 
in  raising  the  weight;  but,  though  equal  in  quantity,  it  can- 
not be  regarded  as  identical  in  the  way  that  the  water  con- 
densed from  steam  is  identical  with  that  which  was  evapo- 
rated to  form  the  steam.  If  measures  showed  it  to  be  less 
in  quantity,  we  could  not  say  there  was  a  destruction  of  an 
identical  something  which  previously  existed,  as  we  could  if 
the  condensed  steam  were  not  equal  to  the  water  eva})()rated. 
Therefore,  while  the  doctrine  of  the  indestructibility  of  force 
is  universally  received  as  a  scientific  principle,  it  can  hardly 
be  claimed  that  induction  has  established  its  absolute  correct- 


504  THE  STELLAR   UNIVERSE. 

ness;  and,  in  a  case  like  the  present,  where  we  s-ie  something 
which  transcends  scientific  explanation,  the  failure  of  the 
widest  induction  may  be  considered  among  the  possible  alter- 
natives. 

The  second  alternative  —  tliat  the  heat  radiated  from  the 
sun  and  stars  continues  its  onward  course  through  space  for- 
ever— is  the  one  most  in  accord  with  our  scientific  concep- 
tions. We  actuallj'  receive  heat  from  the  most  distant  star 
visible  in  our  telescopes,  and  this  heat  has,  according  to  the 
best  judgment  we  can  form,  been  travelling  thousands  of 
years  without  any  loss  whatever.  From  this  point  of  view, 
every  radiation  which  has  ever  emanated  from  the  earth  or 
the  sun  is  still  pursuing  its  course  through  the  stellar  spaces, 
without  any  other  diminution  than  that  which  arises  from  its 
being  spread  over  a  wider  area.  A  ^•ery  striking  presentation 
of  this  view  is,  we  believe,  due  to  some  modern  writer.  If 
an  intelligent  being  had  an  eye  so  keen  that  he  could  see  the 
smallest  object  by  the  faintest  light,  and  a  movement  so  rapid 
that  he  could  pass  from  one  bound  of  the  stellar  system  to  the 
other  in  a  few  years,  then,  by  viewing  the  earth  from  a  dis- 
tance much  less  than  that  of  the  farthest  star,  he  would  see  it 
by  light  which  had  left  it  several  thousand  years  before.  By 
simply  watching,  he  would  see  the  whole  drama  of  human  his- 
tory acted  over  again,  except  where  the  actions  had  been  hid- 
den by  clouds,  or  under  other  obstacles  to  the  radiation  of  light. 
The  light  from  every  human  action  performed  under  a  clear 
sky  is  still  pursuing  its  course  among  the  stars,  and  it  needs 
only  the  powers  wq  have  mentioned  to  place  a  being  in  front 
of  the  ray,  and  let  him  see  the  action  again. 

If  the  hypothesis  now  under  consideration  be  the  correct 
one,  then  the  heau  radiated  by  the  sun  and  stars  is  forever  lost 
to  them.  There  is  no  known  way  by  which  the  heat  thus  sent 
off  can  be  returned  to  the  sun.  It  is  all  expended  in  produc- 
ing vibrations  in  the  ethereal  medium  which  constantly  ex- 
tend out  farther  and  farther  into  space. 

The  third  hypothesis,  like  the  first,  is  a  simple  conjecture 
permitted  by  the  necessary  imperfection  of  our  knowledge. 


THE  SOUBCES  OF  THE  SUN'S  HEAT.  505 

All  the  laws  of  radiation  and  all  our  conceptions  of  space 
lead  to  the  conclusion  that  the  radiant  heat  of  the  sun  can 
never  be  returned  to  it.  Such  a  return  can  result  only  from 
space  itself  having  such  a  curvature  that  what  seems  to  us  a 
straight  line  shall  return  into  itself,  as  has  been  imagined  by  a 
great  German  mathematician  ;*  or  from  the  ethereal  medium, 
the  vibrations  in  which  constitute  heat  being  limited  in  extent ; 
or,  finally,  through  some  agency  as  yet  totally  unknown  to  sci- 
ence. The  first  idea  is  too  purely  speculative  to  admit  of  dis- 
cussion, while  tlie  other  two  suppositions  transcend  our  science 
as  completely  as  does  that  of  an  actual  annihilation  of  force. 

§  3.  'The  Sources  of  the  Sun's  Heat. 

We  may  regard  it  as  good  as  an  observed  fact  that  the  sun 
has  been  radiating  heat  into  void  space  for  thousands  or  even 
millions  of  years,  without  any  apparent  diminution  of  the  sup- 
ply. One  of  the  most  difficult  questions  of  cosmical  physics — 
a  question  the  difficulty  of  which  was  not  seen  before  the  dis- 
covery of  the  conservation  of  force — has  been,  How  is  this  sup- 

*  This  idea  belongs  to  that  transcendental  branch  of  geometry  which,  rising 
above  those  conceptions  of  space  derived  from  our  experience,  investigates  what 
may  be  possible  in  the  relations  of  parts  of  space  considered  in  their  widest  range. 
It  is  now  conceded  that  the  supposed  a  jiriuri  necessity  of  the  axioms  of  geom- 
etry has  no  really  sound  logical  foundation,  and  that  the  question  of  the  limita- 
tions within  which  l»hey  are  true  is  oue  to  be  settled  bj*experience.  l*specially  is 
this  true  of  the  theorem  of  parallels,  no  really  valid  demonstration  either  that  two 
parallel  straight  lines  will  never  meet  or  never  diverge  being  possible.  By  reject- 
ing the  limitations  imposed  upon  our  fundamental  geometrical  conceptions,  yet 
without  admitting  anytiiing  wliicli  positively  contradicts  them,  several  geometrical 
systems  have  been  constructed  in  recent  times,  which  are  included  under  the  gen- 
eral appellation  of  the  non-Euclidian  (remnetry.  The  most  celebrated  and  re- 
markable of  these  systems  is  that  of  Kiemann,  who  showed  that  altiiough  we  are 
obliged  to  conceive  of  space  as  unbounded,  since  no  position  is  possible  which  has 
not  space  on  all  sides  of  it,  yet  there  is  no  necessity  tiiat  we  shall  consider  it  as 
infinite.  It  may  return  into  itself  in  something  the  manner  of  the  surface  of  a 
sphere,  which,  though  it  has  no  boundary,  yet  contains  only  a  finite  number  of 
square  feet,  and  on  wliich  one  who  travels  straight  forward  indefinitely  will  finally 
arrive  at  his  starting-point.  Although  this  idea  of  the  finitude  of  space  transcends 
our  fundamental  conceptions,  it  does  not  contradict  them,  and  the  most  that  ex- 
perience can  tell  us  in  the  matter  is  that,  tliough  space  be  finite,  the  whole  extent 
of  the  visible  universe  can  be  but  a  ven      udl  fraction  of  the  sum  total  of  space. 


506  THE  STELLAR   UNIFEliSE. 

ply  of  heat  kept  up  ?  If  we  calculate  at  what  rate  the  tem- 
perature of  the  sun  would  be  lowered  annually  by  the  radia- 
tion from  its  surface,  we  shall  find  it  to  be  2^°  Fahrenlieit  per 
annum,  supposing  its  specific  heat  to  be  the  same  as  that  of 
water,  and  from  5°  to  10°  per  annum,  if  we  suppose  it  the 
same  as  most  of  the  substances  which  compose  our  globe.  It 
would,  therefore,  have  entirely  cooled  off  in  a  few  thousand 
years  after  its  formation  if  it  had  no  other  source  of  heat 
than  that  shown  by  its  temperature. 

That  the  temperature  could  be  kept  up  by  combustion,  as 
terrestrial  fires  are  kept  up,  is  out  of  the  question,  as  new  fuel 
would  have  to  be  constantly  added  in  quantities  which  cannot 
possibly  exist  in  the  neighborhood  of  the  sun.  But  an  allied 
source  of  heat  has  been  suggested,  founded  on  the  law  of  the 
mechanical  equivalency  of  heat  and  force.  If  a  body  should 
fall  into  the  sun  from  a  great  height,  all  the  force  of  its  fall 
would  be  turned  into  heat,  and  the  heat  thus  produced  would 
be  enormously  greater  than  any  that  would  arise  from  the 
combustion  of  the  falling  body.  An  instance  of  this  law  is 
shown  by  the  passage  of  shooting-stars  and  aerolites  through 
our  atmosphere,  where,  though  the  velocity  rarely  amounts  to 
more  than  forty  miles  a  second,  nearly  all  such  bodies  are  con- 
sumed by  the  heat  generated.  Now,  the  least  velocity  with 
which  a  body  A)uld  strike  tlie*sun  (unless  it  had  been  merely 
thrown  from  the  sun  and  had  fallen  back)  is  about  280  miles 
per  second ;  and  if  the  body  fell  from  a  great  height,  the  ve- 
locity would  be  over  350  miles  per  second.  The  meteoric 
theory  was  founded  on  this  law,  and  is,  in  substance,  that  the 
heat  of  the  sun  is  kept  up  by  the  impact  of  meteors  upon  his 
surface.  The  fact  that  the  earth  in  its  course  around  the  sun 
encounters  millions  of  meteoroids  every  day  is  shown  by  the 
frequency  of  shooting -stars,  and  leads  to  the  result  that  the 
solar  system  is,  so  to  speak,  crowded  with  such  bodies  revolv- 
ing in  all  sorts  of  erratic  orbits.  It  is  therefore  to  be  sup- 
posed that  great  numbers  of  them  fall  into  the  sun ;  and  the 
question  whether  the  heat  thus  produced  can  be  equal  to  that 
radiated  by  the  sun  is  one  to  be  settled  by  calculation.     It  is 


THE  SOURCES  OF  THE  SUN'S  HEAT.  507 

thus  found  that,  in  order  to  keep  up  the  solar  heat,  a  mass  of 
matter  equal  to  our  planet  would  have  to  fall  into  the  sun  ev- 
ery century. 

This  quantity  of  meteoric  matter  is  so  far  beyond  all  rea- 
sonable possibility  that  it  requires  little  consideration  to  show 
that  the  supply  of  solar  heat  cannot  be  thus  accounted  for. 
Only  a  minute  fraction  of  all  the  meteoroids  or  other  bodies 
circulating  through  space  or  revolving  around  the  sun  could 
strike  that  luminary.  In  order  to  reach  the  sun,  they  would 
have  to  drop  directly  to  it  from  space,  or  be  thrown  into  it 
through  some  disturbance  of  their  orbits  produced  by  planet- 
ary attraction.  If  meteors  were  as  thick  as  this,  the  earth 
would  be  so  pelted  with  them  tliat  its  whole  surface  would  be 
made  hot  by  the  force  of  the  impact,  and  all  life  would  be 
completely  destroyed.  While,  then,  the  sun  may,  at  some  past 
time,  have  received  a  large  supply  of  heat  in  this  way,  it  is 
impossible  that  the  supply  could  always  be  kept  up. 

The  Contraction  Theory.  —  It  is  now  known  that  there  is 
really  no  necessity  for  supposing  the  sun  to  receive  heat  from 
any  outward  source  whatever  in  order  to  account  for  the 
preservation  of  his  temperature  through  millions  of  years. 
As  his  globe  cools  off  it  must  contract,  and  the  heat  gener- 
ated by  this  contraction  will  suffice  to  make  up  almost  the  en- 
tire loss.  This*  theory  is  not  only  in  accordance  with  the  laws 
of  matter,  but  it  admits  of  accurate  mathematical  investiga- 
tion. Knowing  the  annual  amount  of  energy  which  tbe  sun 
radiates  in  the  form  of  heat,  it  is  easy,  from  the  mechanical 
equivalent  of  the  heat  thus  radiated,  to  find  by  what  amount 
Jie  must  contract  to  make  it  up.  It  is  thus  found  tliat,  with 
the  present  magnitude  of  the  sun,  his  whole  diameter  need 
contract  but  220  feet  a  year  to  produce  all  the  heat  which  he 
radiates.  This  amounts,  in  round  numbers,  to  a  mile  in  25 
years,  or  four  miles  in  a  century. 

The  question  whether  the  tem|)erature  of  the  sun  will  bo 
raised  or  lowered  by  contraction  depends  on  whether  we  sup- 
pose his  interior  to  be  gaseous,  on  the  one  hand,  or  solid  or 
liquid,  on  the  other.    A  known  principle  of  the  contraction  of 


508  THE  STELLAR   UNIVERSE. 

gaseous' bodies,  and  one  which,  at  first  sight,  seems  paradox- 
ical, is  that  the  more  lieat  such  a  body  loses,  the  hotter  it  will 
become.  By  losing  heat  it  contracts,  but  the  heat  generated 
by  the  contraction  exceeds  that  which  it  had  to  lose  in  order 
to  produce  the  contraction.*  When  the  mass  of  gas  is  so  far 
contracted  that  it  begins  to  solidify  or  liquefy,  this  action 
ceases  to  hold,  and  further  contraction  is  a  cooling  process. 
We  cannot  yet  say  whether  the  sun  has  or  has  not  begun  to 
solidify  or  liquefy  in  his  interior,  and  therefore  cannot  make 
an  exact  estimate  of  the  time  his  heat  will  last.  A  rough 
estimate  may,  however,  be  made  from  the  rate  of  contraction 
necessary  to  keep  up  the  present  supply  of  heat.  This  rate 
diminishes  as  the  sun  grows  smaller  at  such  a  rate  that  in  live 
millions  of  years  the  sun  will  be  reduced  to  one-half  his  pres- 
ent volume.  If  he  has  not  begun  to  solidify  now,  it  seems 
likely  that  he  will  then,  and  his  heat  must  soon  after  begin 
to  diminish.  On  the  whole,  it  is  quite  improbable  that  the 
sun  can  continue  the  radiation  of  sufficient  heat  to  support 
life  on  the  earth  ten  millions  of  years  more. 

The  contraction  theory  enables  us  to  trace  the  past  history 
of  the  sun  a  little  more  definitely  than  that  of  his  future.  He 
must  have  been  larger  a  hundred  years  ago  than  he  is  now  by 
four  miles,  and  yet  larger  in  preceding  centuries.     Knowing 


*  This  curious  law  of  cooling  masses  of  gas  was  discovered  by  Mr.  J.  Homer 
Lane,  of  Washington,  Tiiis  gentleman's  paper  on  the  theoretical  temperature  of 
the  sun,  in  the  American  Journal  of  Science  for  July,  1870,  contains  the  most 
profound  discussion  of  the  subject  with  which  I  am  acquainted.  The  principle  in 
question  may  be  readily  shown  in  the  following  way.  If  a  globular  gaseous  mass 
is  condensed  to  one-half  its  primitive  diameter,  the  central  attraction  upon  any 
part  of  its  mass  will  be  increased  fourfold,  while  the  surface  upon  which  this  at- 
traction is  exercised  will  be  reduced  to  one-fourth.  Hence,  the  pressure  per  unit 
of  surface  will  be  increased  sixteen  times,  while  the  density  will  be  increased  only 
eight  times.  Hence,  if  the  elastic  and  gravitating  forces  were  in  equilibrium  in 
the  primitive  condition  of  the  gaseous  mass,  its  temperature  must  be  doubled  in 
order  that  they  may  still  be  in  equilibrium  when  the  diameter  is  reduced  one-half. 
A  similar  paradox  is  found  in  the  theorem  of  celestial  mechanics — that  the  effect 
of  a  resisting  medium  is  to  accelerate  the  motion  of  a  planet  or  comet  through 
it.  The  effect  of  the  resistance  is  to  make  the  body  approach  the  sun,  and  the 
velocity  generated  by  the  approach  exceeds  that  lost  by  the  resistance. 


THE  SOURCES  OF  THE  SUN'S  HEAT.  509 

the  law  of  his  contraction,  we  can  determine  his  diameter  at 
any  past  time,  just  as  in  the  case  of  the  ninnin<^  clock  the 
height  of  the  weight  dun"  g  preceding  days  can  be  calculated. 
We  can  thus  go  back  to  a  time  when  the  globe  of  the  sun  ex- 
tended out  to  the  orbit  of  Mercury,  then  to  the  orbit  of  the 
earth,  and,  finally,  when  it  filled  the  whole  space  now  occupied 
by  the  solar  system.  We  are  thus  led  by  a  backward  process 
to  the  doctrine  of  the  nebular  hypotliesis  in  a  form  strikingly 
similar  to  that  in  which  it  was  presented  by  Kant  and  La- 
place, although  our  reasoning  is  founded  on  natural  laws  of 
which  those  great  thinkers  had  no  knowledge. 

If  we  take  the  doctrine  of  the  sun's  contraction  as  furnish- 
ing the  complete  explanation  of  the  solar  heat  during  the  whole 
period  of  the  sun's  existence,  we  can  readily  compute  the  total 
amount  of  heat  which  can  be  generated  by  his  contraction 
from  any  assigned  volume.  This  amount  has  a  limit,  however 
great  we  may  suppose  the  sun  to  have  been  in  the  beginning: 
a  body  falling  from  an  infinite  distance  would  generate  only 
a  limited  quantity  of  heat,  just  as  it  would  acquire  only  a  lim- 
ited velocity.  It  is  thus  found  that  if  the  sun  had,  in  the  be- 
ginning, filled  all  space,  the  amount  of  heat  generated  by  his 
contraction  to  his  present  volume  would  have  been  sufficient 
to  last  18,000,000  years  at  his  present  rate  of  radiation.  We 
can  say  with  entire  certainty  that  the  sun  cannot  have  been 
radiating  heat  at  the  piesent  rate  for  more  than  this  period  un- 
less he  has,  in  the  mean  time,  received  a  miraculous  accession 
of  energy  from  some  outside  source.  We  use  the  term  "  mi- 
raculous "  to  designate  any  seeming  incompatibility  with  those 
well  -  ascertained  natural  laws  which  we  see  in  operation 
around  us.  These  laws  teach  us  that  no  body  can  acquire 
heat  except  by  changes  in  its  own  mass  akin  to  contraction  of 
its  parts,  or  by  receiving  it  from  some  other  body  hotter  than 
itself.  The  heat  evolved  by  contraction  from  an  infinite  size, 
or  by  the  falling  of  all  the  parts  of  the  sun  from  an  infinite 
distance,  shows  the  extreme  limit  of  the  heat  the  sun  could 
acquire  from  internal  change,  and  this  quantity,  as  just  stated, 
would  last  only   18,000,000  years.     In   order  that  the  sun 


510  THE  STELLAR   UNIVERSE. 

should  receive  heat  from  another  body,  it  is  not  merely  neces- 
sary that  that  body  should  be  hotter  tlian  the  sun,  but  it  would 
have  to  bo  so  much  hotter  that  the  small  fraction  of  its  radi- 
ant heat  which  reached  the  sun  would  be  greater  than  all  that 
the  sun  himself  radiated.  To  give  an  instance  of  what  this 
condition  requires,  we  remark  that  the  body  must  radiate 
more  heat  than  the  sun  in  the  proportion  that  the  entire  vis- 
ible celestial  sphere  bears  to  the  apparent  angular  magnitude 
of  the  body  as  seen  from  the  sun.  For  instance,  if  its  appar- 
ent diameter  were  twelve  degrees,  it  would  seem  to  fill  about 
^tnnr  P^^i't  of  the  celestial  sphere,  and  in  order  to  warm  the 
sun  at  all  it  would  have  to  radiate  more  than  three  thousand 
times  as  much  heat  as  the  sun  did.  Moreover,  in  order  to  fur- 
nish sufficient  heat  to  last  the  sun  any  given  length  of  time, 
it  would  have  to  stay  in  the  sun's  neighborhood  so  long  that 
the  excess  of  what  the  snn  received  over  what  he  radiated 
would  furnish  a  supply  of  heat  suffiicient  for  that  time.  We 
cannot  suppose  the  sun  to  have  received  even  a  supply  of  a 
thousand  years  of  heat  in  this  way  without  the  most  extrava- 
gant assumptions  respecting  the  vohnne,  the  temperature,  and 
the  motion  of  the  body  from  which  the  heat  was  received — 
assumptions  which,  in  addition  to  their  extravagance,  would 
involve  the  complete  destruction  of  the  planets  by  the  heat  of 
the  body,  and  the  total  disarrangement  of  their  orbits  by  its 
attraction,  if  we  suppose  them  to  have  been  in  any  way  pro- 
tected from  this  heat. 

The  foregoing  computation  of  the  limit  of  time  the  sun  can 
have  been  radiating  heat  is  founded  on  the  supposition  that 
the  amount  of  heat  radiated  has  always  been  the  same.  If 
we  suppose  this  amount  to  have  been  less  formerly  than  now, 
the  period  of  the  sun's  existence  may  have  been  longer,  and 
in  the  contrary  case  it  may  have  been  shorter.  The  amount 
in  question  depends  on  several  causes,  the  effect  of  which  can- 
not be  accurately  computed — namely,  the  magnitude,  temper- 
ature, and  condition  of  the  solar  globe.  Supposing  a  uniform 
radiation,  the  diameter  of  this  globe  was  twice  as  great  nine 
millions  of  years  ago  as  it  is  now.     Its  surface  was  then  of 


SECULAR   COOLING   OF  TUE  EARTH.  511 

four  times  its  present  extent,  so  that,  if  it  was  of  the  same 
nature  and  at  the  Kamc  temperature  as  now,  there  wouhl  have 
been  four  times  the  radiation.  But  its  densitv  would  have 
been  only  one-eighth  as  «>;reat  as  at  present,  and  its  temper- 
ature would  have  been  lower.  These  circumstances  would 
tend  to  diminish  its  radiation,  so  that  it  is  cpiite  possible  that 
the  total  amount  of  heat  radiated  was  no  greater  than  at 
present.  The  probability  would  seem  to  be  on  the  side  of  a 
greater  total  radiation,  and  this  ])robability  is  strengthened  by 
geological  evidence  that  the  earth  was  warmer  in  its  earlier 
ages  than  now.  If  we  reflect  that  a  diminution  of  the  solar 
heat  by  less  than  one-fourth  its  amount  would  probably  make 
our  earth  so  cold  that  all  the  water  on  its  surface  would 
freeze,  while  an  increase  by  much  more  than  one-half  would 
probably  boil  the  water  all  away,  it  must  be  admitted  that  the 
balance  of  (jauses  which  would  result  in  the  sun  radiating  heat 
just  fast  enough  ty  preserve  the  earth  in  its  present  state  has 
probably  not  existed  more  than  10,000,000  years.  This  is, 
therefore,  near  the  extreme  limit  of  time  that  we  can  suppose 
water  to  have  existed  on  the  earth  in  the  fluid  state. 

§  4.  Secular  Cooling  of  the  Earth. 

An  instance  of  a  progressive  loss  of  heat,  second  in  impor- 
tance only  to  the  loss  from  the  sun  itself,  and,  indeed,  con- 
nected with  it,  is  afforded  by  the  secular  cooling  of  the  earth. 
As  we  have  shown  in  a  preceding  chapter,  the  interior  of  the 
earth  is  hotter  than  the  surface,  and  wherever  there  is  such 
a  difference  of  temperature  as  this,  there  must  be  a  conduc- 
tion of  heat  from  the  hotter  to  the  colder  parts.  In  order 
that  heat  may  thus  be  conducted,  there  must  be  a  supply  of 
heat  inside.  The  increase  of  heat  downwards  into  the  earth 
cannot,  therefore,  terminate  suddenly,  but  must  extend  to  a 
great  depth. 

Whatever  view  we  may  take  of  the  question  of  the  earth's 
fluidity,  it  must  be  admitted  that  it  was  hotter  in  former  ages 
than  now.  To  borrow  an  illustration  from  Sir  William  Thom- 
son, the  case  is  much  the  same  as  if  we  should  find  a  hot  stone 


512  THE  STELLA li   UNI VE USE. 

ill  a  iield.  Wc  conld  say,  with  entire  certainty,  that  tlie  stone 
had  been  in  the  fire,  or  some  other  hot  pUicc,  within  a  limited 
period  of  time.  Respecthig  tlie  origin  of  tliis  lieat,  two  hy- 
potheses have  prevailed — one,  founded  on  the  nebular  theory, 
that  the  earth  was  originally  condensed  as  a  molten  mass,  and 
has  not  yet  cooled  off ;  the  other,  that  it  received  its  heat  from 
some  external  source.  The  latter  was  the  view  of  Poisson, 
who  accounted  for  the  increase  of  temperature  by  supposing 
that  the  solar  system  had,  at  some  former  ])eriod,  passed 
through  a  hotter  region  of  space  than  that  in  which  it  is  now 
found.  This  view  is,  however,  now  known  to  be  entirely  un- 
tenable, for  several  reasons.  Space  itself  cannot  be  warm, 
and  the  earth  could  have  derived  heat  only  from  passing  near 
a  hot  body.  A  star  passing- near  enough  to  heat  up  the  earth 
would  have  totally  disarranged  the  ])lanetary  orbits,  by  its  at- 
traction, and  destroyed  all  life  on  the  surface  of  the  globe  by 
its  heat. 

Thus,  tracing  back  the  earth's  heat,  we  are  led  back  to  the 
time  when  it  was  white-hot;  and  then,  again,  to  when  it  was 
enveloped  in  the  liery  atmosphere  of  the  sun  ;  and  again,  wlien 
it  was  itself  a  mass  of  fiery  vapor.  Respecting  the  time  re- 
quired for  it  to  cool  off,  we  cannot  make  any  exact  calcula- 
tion, as  we  have  done  in  the  ease  of  the  sun,  because  the  cir- 
cumstances are  entirely  different.  Owing  to  the  solidity  of  at 
least  the  outer  crust  of  the  earth,  the  heat  which  it  loses  bears 
no  known  relation  to  its  interior  temperature.  In  fact,  were 
we  to  compute  how  long  the  earth  might  have  been  able  to 
radiate  heat  at  its  present  rate,  we  may  find  it  to  be  counted 
by  hundreds  or  thousands  of  millions  of  years.  The  kernel 
of  the  difticulty  lies  in  the  fact  that  when  a  solid  crust  once 
formed  over  the  molten  earth,  there  was  a  sudden  change  in 
the  rate  of  cooling.  As  long  as  the  globe  was  molten,  there 
would  be  constant  currents  between  its  surface  and  the  inte- 
rior, the  cooling  superficial  portion  constantly  sinking  down, 
and  being  replaced  by  fresh  hot  matter  from  the  interior. 
But  when  a  continuous  solid  crust  was  once  formed,  the  heat 
could  reach  the  surface  only  by  conduction  through  the  crust. 


SECULAR  COOLIXa   OF  THE  EARTH.  513 

aiul  tlic  latter,  though  only  a  fow  foot  thick,  would  ojicrate  as 
a  screen  to  prevent  the  further  loss  of  heat.  There  would,  as 
the  crust  cooled,  be  enormous  eruptions  of  molten  matter  from 
the  interior;  hut  these  would  rapidly  cool,  and  thus  help  to 
thicken  the  crust. 

A  fact  not  to  he  lost  sight  of,  and  which  in  some  way  as- 
similates the  earth  to  the  sun,  is  that  of  the  heat  lost  by  the 
earth  by  far  the  greater  part  is  made  up,  not  by  a  lowering 
of  the  temperature  of  the  earth,  but  by  its  contraction.  It  is 
true  that  there  must  be  some  lowering  of  temperature,  but  for 
each  degree  that  the  temperature  is  lowered  there  will  prol)a- 
bly  be  a  hundred  degrees  of  heat  evolved  by  the  contraction 
of  our  globe.  Considering  only  the  earth,  it  is  difficult  to  set 
an  exact  limit  to  the  time  it  may  have  been  cooling  since  its 
crust  was  fortned. 

The  sudflen  change  produced  in  the  radiation  of  a  molten 
body  by  the  formation  of  a  soi'd  crust  over  its  surface  may 
afford  us  some  clue  to  the  probable  termination  of  the  heat- 
giving  powers  of  the  sun.  Whenever  the  latter  so  far  cools 
off  that  a  continuous  solid  crust  is  formed  over  its  surface,  it 
will  rapidly  cease  to  radiate  the  heat  necessarj'  to  support  life 
on  the  globe.  At  its  present  rate  of  radiation,  the  sun  M'ill  be 
as  dense  as  the  earth  in  about  12,000,000  years;  and  it  is 
quite  likely  to  be  long  before  that  time  that  we  are  to  expect 
the  permanent  formation  of  sucli  a  crust. 

The  general  cosmical  theory  which  we  have  been  consider- 
ing accounts  for  the  supposed  physical  constitution  of  Jupiter, 
which  has  been  described  in  treating  of  that  planet.  On  the 
nebular  hypothesis,  as  wa  have  set  it  forth,  the  ages  of  the 
several  planets  do  not  greatly  differ.  The  smaller  planets 
would,  thei'efore,  cool  off  sooner  than  the  larger  ones.  It  is 
possible  that,  owing  to  the  great  masses  of  Jupiter  and  Saturn, 
their  rate  of  cooling  has  been  so  slow  that  no  solid  crust  is  yet 
formed  over  them.  In  tin  case  they  would  appear  self-lumi- 
nous, were  they  not  suri-ounded  by  immense  atmospheres,  filled 
with  clouds  and  vapors,  which  shut  off  a  great  part  of  the 
internal  heat,  and  thus  delay  the  cooling  process. 

34 


514  THE  STELLAR   UNIVERSE. 

§  5.  General  Conclusions  respecting  the  Nebular  Hypothesis. 

It  would  seem  from  what  has  been  said  that  the  widest  in- 
ductions of  modern  science  agree  with  the  speculations  of 
thinking  minds  in  past  ages,  in  presenting  the  creation  of  the 
material  universe  to  our  view  as  a  process  rather  than  act. 
This  process  began  when  the  present  material  universe  was  a 
mass  of  tiery  vapor,  filling  the  stellar  spaces ;  it  is  still  going 
on  in  its  inevitable  course,  and  it  will  end  when  sun  and  stars 
are  reduced  to  dark  and  cold  masses  of  dead  matter.  The 
thinking  reader  will,  at  this  stage  of  the  inquiry,  very  natu- 
rally inquire  whether  this  view  of  the  cosmogony  is  to  be 
received  as  an  established  scientific  fact,  or  only  as  a  result 
which  science  makes  more  or  less  probable,  but  of  the  validity 
of  which  opinions  may  reasonably  differ.  We  consider  that 
the  latter  is  the  more  correct  view.  All  scientific  conclusions 
necessarily  rest  on  the  postulate  that  the  laws  of  nature  are 
absolutely  unchangeable,  and  that  their  operations  have  never 
been  interfered  with  by  the  action  of  any  supernatural  cause ; 
that  is,  by  any  cause  not  now  in  operation  in  nature,  or  op- 
erating in  any  way  different  from  that  in  which  it  has  always 
done.  The  question  of  the  correctness  of  this  postulate  is  one 
of  philosophy  and  common-sense  rather  than  of  science ;  and 
all  we  can  say  in  its  favor  is  that,  as  a  general  rule,  the  bet- 
ter men  understand  it,  the  more  difficulty  they  find  in  doubting 
it.  And  all  we  can  say  in  favor  of  the  nebular  hypothesis 
amounts  to  this :  that  the  operations  of  nature,  in  their  widest 
range,  when  we  trace  them  back,  seem  to  lead  us  to  it,  as 
the  mode  of  running  of  the  clock  leads  to  the  conclusion  that 
it  was  once  wound  up. 

llelmholtz,  Thomson,  and  others  have,  as  we  have  explain- 
ed, made  it  evident  that  by  tracing  back  the  cooling  processes 
we  now  see  going  forward  in  nature,  we  are  led  to  a  time 
when  the  planets  were  enveloped  in  the  fiery  atmosphere  of 
the  sun,  and  were  therefore  themselves  in  a  molten  or  vapor- 
ous form.  But  the  reverse  problem,  to  show  that  a  nebulous 
mass  would  or  might  condense  into  a  system  possessing  the 


COXCLUSIONS  liFSrECTIXG  THE  NEBULAE  HYPOTHESIS.    515 

wonderful  symmetry  of  our  solar  system — the  planets  revolv- 
ing round  the  sun,  and  the  satellites  round  their  })rimaries 
in  nearly  circular  orbits — has  not  been  solved  in  a  manner  at 
all  satisfactory.  We  have  seen  that  Kant's  ideas  were  in  sonic 
respects  at  variance  with  the  laws  of  mechanics  which  have 
since  been  discovered.  Laplace's  explanation  of  how  the 
planets  might  have  been  formed  from  the  atmosphere  of  the 
sun  is  not  mathematical  enough  to  be  conclusive.  In  the  ab- 
sence of  a  mathematical  investigation  of  the  subject,  it  seems 
more  likely  that  the  solar  atmosphere  would,  under  the  condi- 
tions supposed  by  Laplace,  condense  into  a  swarm  of  small 
bodies  like  the  asteroids,  tilling  the  whole  space  now  occupied 
by  the  planets.  Again,  when  we  examine  the  actual  nebulae, 
we  find  very  few  of  them  to  present  that  synnnetry  of  outline 
which  would  lead  to  their  condensation  into  a  system  so  sym- 
metrical as  that  to  which  our  planet  belongs.  Tlie  double 
stars,  revolving  in  orbits  of  every  degree  of  eccentricity,  and 
the  rings  of  Saturn,  composed  apparently  of  a  swarm  of  small 
particles,  offer  better  examples  of  what  we  should  expect  from 
the  nebular  hypothesis  than  do  the  planets  and  satellites  of  our 
system. 

These  difficulties  raav  not  bo  insurmountable.  The  greatest 
of  them,  perhaps,  is  to  show  how  a  ring  of  vapor  surrounding 
the  sun  could  condense  into  a  single  planet  eiu!ircled  by  satel- 
lites. Tiie  conditions  under  which  such  a  result  is  possible 
require  to  be  investigated  mathematically.  At  the  present 
time  we  can  oidy  say  that  the  nebular  hypothesis  is  indicated 
by  the  general  tendencies  of  the  laws  of  nature ;  that  it  has 
not  been  proved  to  be  inconsistent  with  any  fact;  that  it  is 
almost  a  necessary  consequence  of  tlie  only  theory  by  M'hich 
we  can  account  for  the  origin  and  conservation  of  the  sun's 
heat ;  but  that  it  rests  on  the  assumption  that  this  conservation 
is  to  be  explained  by  the  laws  of  nature,  as  we  now  see  them 
in  operation.  Should  any  one  be  sceptical  as  to  the  sufficiency 
of  these  laws  to  account  for  the  present  state  of  things,  science 
can  furnish  no  evidence  strong  enough  to  overthrow  his  doubts 
until  the  snn  shall  be  found  growing  smaller  by  actual  mcas- 


516  THE  STELLAR   UNIVERSE. 

urenient,  or  the  riebulfE  be  actually  seen  to  condense  into  stars 
and  systems. 

§  6.  The  Pluralitij  of  Worlds. 

When  we  contemplate  the  planets  as  worlds  like  our  own, 
and  the  stars  as  suns,  each,  perhaps,  with  its  retinue  of  attend- 
ant planets,  the  idea  naturally  suggests  itself  that  other  planets 
as  well  as  this  may  be  the  abode  of  intelligent  beings.  The 
question  whether  other  planets  are,  as  a  general  rule,  thus 
peopled,  is  one  of  the  highest  interest  to  us,  not  only  as  in- 
volving our  place  in  creation,  but  as  showing  us  what  is  really 
greatest  in  the  universe.  Many  thinking  people  regard  the 
discovery  of  evidences  of  life  in  other  M'orlds  as  the  great  ul- 
timate object  of  telescope  research.  It  is,  therefore,  extreme- 
ly disappointing  to  learn  that  the  attainment  of  any  direct 
evidence  of  such  life  seems  entirely  hopeless  —  so  hopeless, 
indeed,  that  it  has  almust  ceased  to  occupy  the  attention  of 
astronomers.  The  spirit  of  modern  science  is  wholly  adverse 
to  speculation  on  questions  for  the  solution  of  which  no  scien- 
tific evidence  is  attainable,  and  the  common  answer  of  astron- 
omers to  all  questions  respecting  life  in  other  woi'lds  would 
be  that  they  knew  no  more  on  the  subject  than  any  one  else, 
and,  having  no  data  to  reason  from,  had  not  even  an  opinion 
to  express.  Still,  in  spite  of  this,  many  minds  will  speculate; 
and  although  science  cannot  answer  the  great  question  for  us, 
she  may  yet  guide  and  limit  our  s})eculations.  It  may,  there- 
fore, not  be  unprofitable  to  show  within  what  limits  specula- 
tion may  not  be  discordant  with  the  generalizations  of  science. 

First,  WQ  see  moving  round  our  sun  eight  large  planets,  on 
one  of  which  we  live.  Our  telescopes  show  us  other  suns,  in 
such  numbere  that  they  defy  count,  amounting  certainly  to 
many  millions.  Are  these  suns,  like  our  own,  centres  of  plan- 
etary systems?  If  our  telescopes  could  be  made  powerful 
enough  to  show  such  planets  at  distances  so  immense  as  those 
of  the  fixed  stars,  the  question  would  at  once  be  settled ;  but 
all  the  planets  of  our  system  wouh!  disappear  entirely  from 
the  reach  of  the  most  powerful  telescopes  we  can  ever  hope  to 


THE  PLURALITY   OF  WORLDS.  517 

make  at  a  distance  far  less  than  that  which  separates  us  from 
the  nearest  fixed  star.  Observation  can,  therefore,  aftord  us 
no  information  on  the  subject.  We  must  have  recoui'sc  to 
cosmological  considerations,  and  these  may  lead  to  the  con- 
clusion that  if  the  whole  universe  condensed  from  a  nebulous 
mass,  the  same  cause  which  led  our  sun  to  be  surrounded  by 
planets  Avould  operate  in  the  case  of  other  suns.  But  we  have 
just  shown  that  the  symmetry  of  form  and  arrangement  seen 
in  our  system  is  something  we  could  rarely  expect  to  result 
from  the  condensation  of  masses  so  irregidar  as  those  which 
make  up  the  large  majority  of  the  nebuhe,  while  the  irreg- 
ular orbits  of  the  double  stars  show  us  what  we  should  rather 
expect  to  be  the  rule.  It  is,  therefore,  quite  possible  that  reti- 
nues of  planets  revolving  in  circular  orbits  may  be  rare  excep- 
tions, rather  than  the  rule,  among  the  stars. 

Next,  granting  the  existence  of  planets  without  number, 
what  indications  can  we  have  of  their  habitability  ?  There 
is  one  planet  besides  our  own  for  which  the  telescope  settles 
this  point — namely,  the  moon.  This  body  has  neither  air  nor 
water,  and,  consequently,  nothing  on  which  organic  life  can 
be  supported.  The  speculations  sometimes  indulged  in  re- 
specting the  possible  habitability  of  the  other  side  of  the 
mooii,  which  we  can  never  see,  are  nothing  more  than  i>lays 
of  the  imagiiuition.  The  prinuiry  planets  are  all  too  distant 
to  enable  us  to  form  any  certain  judgment  of  the  nature  of 
their  surfaces,  and  the  little  we  can  see  indicates  that  their 
constitution  is  extremely  varied.  Mars  has  every  ai)pearance 
of  being  like  our  earth  in  many  particulars,  and  is,  therefore, 
the  planet  which  we  should  most  expect  to  find  inhabited. 
Most  of  the  other  planets  give  indications  of  being  surround- 
ed by  immense  atmospheres,  filled  with  clouds  and  vapors, 
through  which  sight  camiot  penetrate,  and  we  can  reach  no 
certain  knowledge  of  what  may  be  under  these  clouds.  On 
the  whole,  we  nuiy  consider  the  chances  t  Le  decidedly 
against  the  idea  that  any  considerable  fraction  of  the  heav- 
enly bodies  are  fitted  to  be  the  abode  of  such  animals  as  we 
have  on  the  earth,  and 'that  the  number  of  them  which  have 


518  THE  STELLAR   UNIVERSE. 

the  requisites  for  supporting  civilization  is  a  very  small  frac- 
tion indeed  of  the  whole. 

This  conclusion  rests  on  the  assumption  that  the  conditions 
of  life  are  the  same  in  other  worlds  as  in  our  own.  This  as- 
sumption may  be  contested,  on  the  ground  that  we  can  set  no 
limits  to  the  power  of  the  Creator  in  adapting  life  to  the  con- 
ditions which  surround  it,  and  that  the  immense  range  of  adap- 
tation on  our  globe — some  animals  living  where  others  are  im- 
mediately destroyed — makes  all  inferences  founded  on  the  im- 
possibility of  our  earthly  animals  living  in  the  planets  entirely 
inconclusive.  The  only  scientific  way  of  meeting  this  argu- 
ment is  to  see  whether,  on  our  earth,  there  are  any  limits  to 
the  adaptability  in  question.  A  cursory  examination  shows 
that  while  there  are  no  well-detined  limits  to  what  may  be 
considered  as  life,  the  highei*  forms  of  animal  life  are  very 
far  from  existing  equally  under  all  conditions,  and  the  high- 
er the  form,  the  more  restricted  the  conditions.  We  know 
that  no  animal  giving  evidence  of  self-consciousness  is  devel- 
oped except  under  the  joint  influence  of  air  and  water,  and 
between  certain  narrow  limits  of  temperature;  that  only  forms 
of  life  which  are  intellectually  very  low  are  developed  in  the 
ocean  ;  that  there  is  no  adapting  power  exercised  by  nature  on 
our  globe  whereby  man  can  maintain  a  high  degree  of  intel- 
lectual or  bodily  vigor  in  the  polar  regions ;  that  the  heats  of 
the  torrid  zone  also  impose  restrictions  upon  the  development 
of  our  race.  The  conclusion  wliich  we  may  draw  from  this 
is  that,  if  great  changes  should  occur  on  the  surface  of  our 
globe,  if  it  should  be  cooled  down  to  the  temperature  of  the 
poles,  or  heated  \i\)  to  that  of  the  equator,  or  gradually  be  cov- 
ered with  water,  or  deprived  of  its  atmosphere,  the  higher  pres- 
ent forms  of  animal  life  would  refuse  to  adapt  themselves  to 
the  new  state  of  things,  and  no  new  forms  of  life  of  equal  ele- 
vation would  take  the  place  of  those  destroyed  by  the  change. 
There  is  not  the  slightest  reason  for  believing  that  anything 
more  intelligent  than  a  fisli  would  ever  live  under  water,  or 
anything  more  intellectual  than  the  Esquimaux  ever  be  sup- 
ported in  regions  as  c<jld  as  the  poles.     If  we  apply  this  con- 


THE  I'LURALITV  OF  WORLDS.  51i) 

sideration  to  the  question  wliich  now  occupies  us,  we  are  led 
to  the  conchision  that,  in  view  of  the  inunense  diversity  of 
conditions  which  probably  prevails  in  the  universe,  it  would 
be  only  in  a  few  favored  spots  that  we  should  expect  to  find 
any  very  interesting  development  (     life. 

An  allied  consideration  will  lead  us  to  nearly  the  same  con- 
clusion. Enthusiastic  M'riters  not  only  sometimes  people  the 
planets  with  inhabitants,  but  calculate  the  possible  population 
by  the  number  of  square  miles  of  surface,  and  throw  in  a  lib- 
eral supply  of  astronomers  who  scan  our  earth  with  powerful 
telescopes.  The  possibility  of  this  it  would  be  presumption 
to  deny ;  but  that  it  is  extremely  improbable,  at  least  in  the 
case  of  any  one  planet,  may  be  seen  by  reflecting  on  the  brev- 
ity of  civilization  on  our  globe,  when  compared  with  the  exist- 
ence of  the  globe  itself  as  a  planet.  The  latter  has  probably 
been  revolving  in  its  orbit  ten  millions  of  years;  man  has 
probably  existed  on  it  less  than  ten  thousand  years;  civiliza- 
tion less  than  four  thousand  ;  telescopes  little  more  than  two 
hundred.  Had  an  angel  visited  it  at  intervals  of  ten  thousand 
years  to  seek  for  thinking  beings,  he  would  have  been  disap- 
pointed a  thousand  times  or  more.  Ileasoning  from  analogy, 
we  are  led  to  believe  that  tlie  same  disappointments  might 
await  him  who  should  now  travel  from  planet  to  planet,  and 
from  system  to  system,  on  a  similar  search,  until  he  had  exam- 
ined many  thousand  planets. 

It  seems,  therefore,  so  far  as  we  can  reason  from  analogy, 
that  the  probabilities  are  in  favor  of  only  a  very  small  frac- 
tion of  the  planets  being  peoi)led  with  intelligent  beings. 
But  when  we  reflect  that  the  possible  mimber  of  the  planets 
is  counted  by  hundreds  of  millions,  this  small  fraction  may 
be  really  a  very  large  number,  and  among  this  mnnber  many 
may  be  peopled  by  beings  much  higher  than  ourselves  in  the 
intellectmil  scale.  Here  we  may  give  free  rein  to  our  imagi- 
nation, with  the  moral  certainty  that  science  will  supply  noth- 
ing tending  either  to  prove  or  to  disprove  any  of  its  fancies. 


ADDENDUM  TO  PART  III.,  CHAPTER  11. 

As  this  work  is  passing  through  tho  iiress,  Professor  Henry  Draper,  of 
Now  York,  lias  nia<lo  an  addition  to  the  theory  of  the  solar  spectrnni  which 
can  hardly  fail  to  add  a  (piite  uew  feature  to  tho  spectral  analysis  of  tho 
siiu  aud,  perhaps,  to  that  of  other  heavenly  bodies.  Hitherto  the  solar 
spectrum  h.as  been  universally  considered  as  a  continuous  one,  like  that 
from  a  glowing  solid,  crossed  by  dark  lines  produced  by  the  absorption  of 
vapors  surrouiuliug  the  sun.  This  is  tho  view  explained  ou  page  225. 
Professor  Draper's  i)oint  is  that,  in  addition  to  this,  the  spectrum  is  crossed 
by  the  bright  lines  and  bauds  arising  from  glowing  gases,  and  that  these 
lines  admit  of  being  recognized  in  certain  i)arts  of  the  sj)ectruni  if  the 
proper  steps  are  taken  to  bring  them  out.  That  bright  lines  might  well 
exist  in  the  spectrum  r^o  one  would  deny,  because  tho  gases  of  the  chro- 
mosphere must  produce  them.  lint  it  has  always  been  suppost^d  that  they 
must  bo  so  excessively  faiut  as  to  be  entirely  invisible  when  projected  on 
the  spectrum  of  tho  sun  itself,  and  so  no  one  is  known  to  have  sought  for 
them  with  especial  care.  Dr.  Draper's  cour.se  has  been  to  i)hotograph 
side  by  side  the  solar  spectrum  between  the  lines  G  and  H,  and  tho  corre- 
sponding part  of  tho  spectrum  of  oxygen  rendered  liimiu'  is  by  the  elec- 
tric spark.  Tho  result  is  that  out  of  thirteen  bright  lines  of  oxygen,  some 
of  them  double  or  treble,  nearly  all  have  corresponding  lines  in  the  solar 
spectrum.  Tho  coincidence  is  so  striking  that  it  seems  hardly  po.ssiblo 
to  avoid  the  conclusion  that  a  considerable  part  of  the  violet  light  of  the 
sun's  spectrum  arises  from  glowing  oxygen  in  the  i)hotosphero. 

The  reason  why  these  lines  are  brought  out  here  when  they  are  not 
found  in  other  parts  of  tho  spectrum  is  to  be  found  in  the  extreme  faint- 
uess  of  the  violet  part  of  tho  continuous  8i»ectrum,  whereby  the  bright 
liiu;s  are  not  obscured  by  the  dazzling  brilliancy  of  the  background  of  con- 
tinuous spectrum.  If  it  bo  asked  why  the.se  bright  lines  have  not  been 
noticed  before,  the  answer  is,  that  tho  dark  lines  are  hero  so  broad  and 
nuMierous  as  to  cut  up  the  continu(ms  spectrum  into  very  narrow  lines  of 
very  irregular  brightness,  besides  which  absorption  bands  or  half  shades 
are  nuinerou,s.  Again,  the  lines  of  oxygen  do  not  appear  to  be  so  narrow 
and  sharply  defined  as  those  of  tho  metallic  vapors,  aud  this  makes  it  more 
diflicult  to  distinguish  tlu'in  from  spaces  between  tho  dark  bands. 

The  full  conlirmatioii  of  this  discovery  must  be  sought  for  in  careful 
measures  of  the  relative  brilliancy  of  tho  oxygen  lines  in  various  parts  of 
tho  spectrum,  in  order  to  determine  whether  the  violet  lines  are  bright 
enough  to  be  seen  on  tho  background  of  continuous  spectrum,  and  in  a 
more  minute  study  of  tho  .solar  six'ctrum  in  this  region.  It  may  tlum  well 
rank  as  the  most  important  advance  in  spectrum  analysis  since  Lockyer 
aud  Jansseu  discovered  tho  spectrum  of  the  solar  protuberances. 


APPENDIX. 


I. 

LIST  OF  THE  PRINCIPAL  GREAT  TELESCOPES  OF  THE  WORLD. 

A.  Jiijlicthi;/  Tihscopcs. 


Owner,  and  rinco. 


Tiio  Eiirl  of  Rosse,  Parsonstown,  ) 
Ireland \ 

Mr.  Lassell,  Maidenhead,  Eng-  } 
land i|' 

The  Observatory  of  Melbourne,  / 
AustraUa ) 

The  Oljscrvatory  of  Paris 

Tlic  Earl  of  Rosse,  Parsonstown,  \ 

Ireland J 

Professor  Henry  Draper,  Dobbs  / 

Ferry,  Xew  York \ 

The    Observatory    of    Toulouse,  / 

Franee \ 

The   Observatory  of   Marseilles,  | 

Franee ^ 

Mr.    Lassell,    Maidenhead,   Eng-  ) 

land V 


Coiistructiuii.* 

Ajicrture. 

When  built,  imd  by  whom. 

Newtonian. 

G  feet. 

Earl  of  R.,  1844. 

Newtonian. 

■1  feet. 

Mr.  Lassell,  f  180(1 

Cassegr. 

4  feet. 

Mr.  Grubb,  1870. 

Newt.,  S.  CJ. 

47  in. 

i  M.  Martin  and  M. 
I      Eiehens,  1875. 

Newtonian. 

3  feet. 

The  owner. 

Cass.,  S.  G. 

28  in. 

The  owner. 

S.G. 

31.5  in. 

M.  Foucault. 

S.G. 

SI. 5  in. 

\  M.FoueaultandM. 
(       Eiehens. 

Newtonian. 

2  feet. 

The  owner. 

B.  lic/racfhif/  Tchm 

1JXS. 

Owner,  ami  Plnce. 

Aperture. 

Maker,  ami  Date. 

U  S.  Naval  Observatory,  Washington 

2t)  in. 
2(')  in. 
25  in. 
lit  in. 
18.5  in. 
18  in. 
15  in. 

\  A.  Clark  and  Sons, 

"(       1873. 

Mr.  (Jnil)b,  1877. 

\  T.  Cooke  and  Sons, 

\      1870. 

Merz  and  Mahler. 

\  A.  Clark  and  Sons, 

"/       18(i2. 

Mr.  Fitz,  of  N.  Y. 

\  Merz  and   Mahler, 

\       1843. 

The  Imnerial  Observatorv.  Viennat 

Mr  R  S  Newall.  Gateshead.  Endand 

The  Observatory  of  Strasburg,  Gerinany:j: 

The  Observatorv  of  ChicaiTo 

Mr  Van  der  Zee  Buffalo  New  York 

The   Observatory  of  Harvard  College,  Cam-  \ 
bridge.  Mass S 

•  In  this  column,  "Casi^egr."  signifles  the  Casseiinmian  construction,  described  ou  page 
124.    S.  G.  Piunifles  tliat  tlie  niiiTor  is  of  .silvered  i,'Iass. 
t  Mr.  Liissell's  Ibur-foot  telci^cope  is,  the  writer  believes,  dismantled. 
t  These  telescopes  are  still  untiuibhud. 


522 


APPENDIX. 


Owner,  and  Pliice. 


The  Royal  Observatory,  Pulkowa,  Russia 

Mr.  William  Iluj^fjjins,  London,  Enj^land* 

liord  Lindsay,  Aberdeen,  Scotland 

Tiie  ()l)servatory  of  Lisbon,  Portugal 

The  Observatory,  Markrce  Castle,  England. . . . 

Ilaniilton  College,  Clinton,  New  York 

Tlie  Paris  01)servat()ryf 

Tiie  Allegheny  Observatory,  Pennsylvania  .... 

Mr.  L.  M.  Rutherfurd,  \e>v"  York . .' 

The  Dudley  Observatory,  Albany,  New  York. . . 

The    Royal    Observatory,    Greenwich,    Eng-  \ 
land| \ 

Michigan  University,  Ann  Arbor 

Vassar  College,  Poughkeepsie,  New  York 

The  Physical  Observatory,  Oxford,  England.  . . 

The  Imperial  Observatory,  Vienna , 

The  Cambridge  Observatory,  England 

The  Royal  Observatory,  Dui)lin 

Professor  Henry  Draper,  Dobbs  Ferrv,  New  / 
York ■. f 

The  Pritchctt  Listitute,  Glasgow,  Missouri 

Mr.  S.  V.  White,  Brooklyn,  New  York 

The  Radeliffe  Observatory,  Oxford,  England . . .  , 

The  Observatory,  Hothkamp,  Germany 

The  Observatory,  {'ordo\a.  South  America 

The  Observatory,  Munich,  (iermany 

The  Observatory,  Copenhagen,  Denmark 

The  Observatory  of  Cincinnati,  Oiiio 

Middlctown  University,  Connecticut 


Aperture. 


15  in. 

15  in. 
15  in. 
14.8  in. 
14  in. 
i;5.5  in. 
13  in. 
13  in. 
13  in. 
13  in. 


12.5  in. 

12.5  in. 

12.3  in. 

12.2  in. 

12  in. 

1 2  in. 
12  in. 

12  in. 

1 2  in. 

12  in. 
12  in. 
ll.Y  in. 
11.2  in. 
11  in. 
11  m. 
11  in. 
11  in. 


Mttkcr,  anil  Date. 


I  Merz  and  Mahler, 
I       184(1. 
Mr.  (Jrubb. 
Mr.  Grubb. 
Merz  and  Mahler. 


Mr.  Spencer. 
M.  Eichens. 


Sons, 


and 


The  owner. 
Mr.  Fitz,  of  N.  Y. 
f  Merz     and 
J       18  CO. 
I  Troughton 
[^     Sinnns. 
Mr.  Fitz,  of  N.  Y. 
\  Mr.  Fitz,  of  N.  Y. 
]  A.  Clark  and  Sons. 
Mr.  (Jiulib. 
(  A.  Clark  and  Sons, 
'{       1876. 
M.  Cauchoix. 
M.  Cauchoix. 
j  A.  Clark  and  Sons, 
]       1876. 

\  A.  Clark  and  Sons, 
\      1876. 
A.  Clark  and  Sons. 
M.  Cauchoix. 
Schroedev. 
Mr.  Fitz,  of  N.  Y. 
Merz. 
Merz. 
Merz. 
A.  Clark  and  Sons. 


Besides  these,  the  following  telescopes  are  projected :  A  reflector  of 
seveu  or  eight  feet  aperture,  by  Grubb  of  Dublin,  for  the  Lick  Observa- 
tory of  California,  and  a  refractor  of  28  or  29  iuches  aperture,  by  Alvau 
Clark  and  Sous,  for  Yale  College. 


*  This  telescope  belongs  to  the  Royal  Society,  but  is  iu  possession  of  Mr.  Iluggius. 

t  The  object-glass  is  an  old  one,  but  the  mounting  is  new,  by  Eichens. 

t  The  object-glass  is  by  Merz,  of  Munich,  the  mounting  by  Troughtou  and  Sinims. 


LIST  OF  THE  MORE  UEMARKABLE  DOUBLE  STABS.    523 


II. 


LIST  OF  THE  MORE  REMARKAHLE  DOUBLE  STARS. 

COMPILKD    BV    S.   W.   HURNHAM. 


Niime. 

Ri(;lit  A«oen. 

iHSd. 

Declination 

IHWl. 

I'osili'n 
Ani;le. 

Distance. 

Ma^n 

itudes. 

Notes. 

H.    M.     S. 

o           t 

a 

35  Piscium  . . . 

8  47 

8     St 

140.8 

11.53 

6.2 

7.8 

j  White, 2.  Palc-whito: 
j      violet,  Smyth. 

38         "       ... 

11    13 

8   12 

237.0 

4.59 

7.0 

8.0 

42         "       ... 

10   13 

12  40 

338.0 

20.73 

6.8 

10.7 

(Yellow,  hlue-nicen, 
\       Hersehel. 

51          "       ... 

20   12 

0   18 

82.3 

27.42 

5.0 

9.0 

White:  ashy. 

55          "       ... 

33  3t) 

20  47 

192.7 

0.37 

5.0 

8.2 

j  Yellow  :  deep  -  red, 
]      Demhowski. 

1]  CassiopciP. .  . 

41  43 

57  11 

140.0 

5.86 

4.0 

7.0 

Yellow  :  jmi'ple. 

3(»  Aiulronit'dic. 

48  32 

22  50 

358.0 

1.34 

0.2 

6.8 

IJinaiT,  349.1  years. 

ip  I'Lsc'iuin  .... 

1     7   14 

23   57 

227.5 

7.98 

4.7 

10.1 

White:  blue." 

42  Coti 

13  41 

-1     8 

351.4 

1.25 

•).2 

7.2 

Poliiri." 

13  45 

88  40 

210.1 

18.27 

2.0 

O.o 

£  Sculptoris.  .  . 

40     1 

-25  30 

09.0 

5.53 

0.0 

](•.() 

Wiiite :  dull  red. 

a  Piscium  .... 

55  50 

2   11 

322.2 

3.12 

2.8 

3.0 

y  AndroniediC . 

50  32 

41  45 

02.4 

10.33 

3.0 

5.0 

(  Yellow  :  blue.  B 
1      a<:aiii  d()ul)le,  0".5. 

I  Trian<^uli..  .  . 

2     5  25 

20  45 

80.5 

3.08 

5.0 

6.4 

Yello'v  ;   blue. 

t  CassiopciO .  . . 

19   10 

60  52 

205.1 

2.01 

4.2 

7.1 

A  and  R.  [ 

<    •    •    ■ 

■    •    *    • 

107.3 

7.02 

8.1 

A  and  C.  f 

84Ceti 

35     4 

-1    12 

324.7 

4.03 

0.0 

9.2 

Yellow  :  ashy. 

y  Ceti 

37     5 

2  44 

289.2 

2.07 

3.0 

6.8 

Yellow  ;  ))hie. 

£  Arietis 

52  21 

20  52 

201.9 

1.20 

5.7 

6.0 

Hillary. 

i  Lijjiit  -  fireen  :  ashy. 
}  Other  smal'  stars 
(      ill  the  Held. 

^  Persei 

3  40  35 

31  32 

207.6 

12.47 

2.7 

9.3 

£  Poivsei 

4'.)  48 

30  40 

9.2 

8.81 

3.1 

8.3 

Pale-white  :  lilae. 

31)  Eriduui 

4     8  41 

-10  33 

153.7 

0.20 

0.0 

9.1 

Yellow  :  l)lue. 

0  Taiiri 

12  58 

27     4 

245.5 

53.78 

5.0 

8.0 

Red:  Iduish. 

p  Orioni;* 

5     7     1 

2  43 

03.4 

7.05 

4.7 

8.5 

Yellow :  blue. 

Jj       "       

8  40 

—  8  20 

198.8 

9.14 

1.0 

8.0 

-)^     " 

10  32 

3  20 

28.1 

31.71 

5.0 

7.0 

J       "       ..... 

18  27 

-2  30 

83.8 

1.11 

4.0 

5.0 

Discovered  by  Dawes. 

\       "       

28  32 

9  51 

40.3 

4.23 

4.0 

0.0 

Yellow  ;  jjurple. 

0>      "       

20  23 

-5  28 

■ 

\  Scxtujile.  In  the  {^reat 
}      nebula  ol  Orion. 

<T          "          

32  43 

—  2  4( 

230.5 

11.00 

4.1 

10.3 

A  and  H.  I 

84.5 

12.86 

7.5 

A  and  0.  S 

Z  Ononis 

34  42 

-2     ( 

151.3 

2.55 

2.0 

5.7 

Yellow :  light-purple. 

1 1  Monoceroti.s. 

0  23     0 

-0  57 

130.0 

7.25 

5.0 

5.5 

A  and  15.  / 

101.7 

2.40 

13.0 

H  and  ('.  ( 

1 2  Lyncis 

35  38 

59  34 

153.7 

1.53 

K  0 

6.1 

A  and  15.  [ 





304.2 

8.67| 

7.4 

A  and  C.  f 

524 


A  r  VEND  IX. 


Name. 


5()  Aiirif^ie . . . 
fi  Canis  Miij. . 
t^  (u'lniiioruiu 

Castor 

5  Navis 

^  Caneri 


SS  Lync'is. .  .  . 
y  Leonis  .... 
35  St'xtantis . 
5  Ursa;  Maj.  . 
tl5  Ursffi  Maj. , 

f  Coina3 

'24     " 

y  Virginis  . .  . 
8.5  ComiL' .... 


(J  Seriiontis  . 
?  Libric .... 


/3  Cysiii... 
K  Sa'j^ittiP.. 
£  Dracoiiis . 
9  Sagittic. 
49  VyffM . . 
£  Equuk'i . . 


Ul);)it  Ascun. 

IHMd. 


84  Virfjinis . . . 
'C  Boijtis 

£  "        

^  "       

44     "     

/^       "     


Antaros 

36  Ophiuchi  . . 
a  Herculis . . . . 
p  "  .... 
7<^  Ophiut'hi  . . 
£'  Lvra; 


12  Aquarii. . . . 

(U  Cysni 

j3  Cephei 

41  A(iuarii. . .  . 
53  "  . . . . 
^  "     .... 

,//  "     

(T  Cassiopeae  .  . 


H.    M. 

38 


s. 
5 

50  3(1 
12  57 
26  57 
42  19 
5   19 


10 


11 


12 


il 
13 

37 
11 

48 
58 
29 
35 
47 


If) 

17 


22 

"l 

9 

19 


18 


19 


20 


23 

20 

7 

48 

51 

8 

6 

3(1 

23 


13  37     2 

14  35  25 
39  45 
45  51 
59  51 

15  19  58 

29     5 
57  40 


59 
10 
33 


59  23 
40  22 
40  24 
40  38 
25  53 
43  39 
48  34 
4  39 
30  11 
53     5 

57  44 


21 


22 


23 


1 

27 

7 

20 


14 

0 

40 

3 


22  39 
9  35 

52  50 


DuclliiHtlon  roaiti'i] 
INNO,  Aii)(li'. 


43 
-13 

22 

32 
-11 

18 

37 
20 
5 
32 
47 
22 
19 
-0 
21 

4 

14 
27 
19 
48 
37 

10 
-11 

-20 

-20 

14 

37 

2 

39 
39 
37 
27 
18 
09 
20 
31 
3 

-0 

38 

70 

-21 

-17 

-0 

-9 

55 


42 
53 
12 

9 
54 

1 

19 

27 

23 

13 

9 

8 

2 

47 
54 

9 
15 

35 
30 

7 
48 

50 
3 

10 
25 
32 
15 
33 
33 
29 
29 
42 
51 
58 
33 
53 
50 

18 
8 
2 

40 
21 
38 
44 
5 


o 

17.1 
343.5 
190.9 
239.3 

17.5 
130.1 
132.0 
240.2 
111.2 
240.5 
317.0 

36.4 
240. 0 
271.9 
159.3 

25.3 
124.7 
235.3 
303.2 
320.0 
301.0 
239.8 
171.9 
141.9 
190.9 
173.1 

70.3 
208.7 
227.3 
118.5 
307.2 

83.7 

26.0 
155.2 
149.7 

55.7 
312.8 
354.5 
326.7 

49.4 
283.9 

76.2 
189.6 
115.6 
250.0 
119.4 
304.5 
334.5 
312.2 
323.4 


DiatAncu. 


55.38 
3.22 
7.14 
5.49 
3.32 
0.74 
5.'*  3 
2.69 
3.18 
0.72 
1.09 
3.71 
3.73 

20.42 
4.77 
1.43 

28.00 
3.39 
1.02 
2.03 
5.44 
4.80 
108.40 
0.09 
2.50 
l.OO 
7.05 
3.40 
5.55 
4.05 
3.00 
3.48 
3.03 
2.57 

43.71 

34.29 
8.49 
2.79 

11.40 
2.74 
0.00 

10.83 
2.06 

19.55 

13.57 
4.08 
8.20 
3.40 

49.03 
3.01 


Maitnltuilei. 


o.o 
4.7 
3.2 
2.7 
5.3 
5.0 
I 
4.0 
2.0 
0.1 
4.0 
0.0 
6.0 
1.7 
3.0 
5.0 

5.8 
3.5 
3.0 
4.7 
5.2 

0.7 
3.0 
4.9 

1.0 
0.0 
3.0 
4.0 
4.1 
4.6 
4.9 
4.2 
3.0 
5.7 
4.0 
(i.O 
6.0 
5.2 

5.0 
5.3 
3.0 
6.0 
6.0 
4.0 
4.5 
5.4 


5.5 


9, 

8. 
8. 
8, 

7, 
5, 

6, 
3, 

7, 
4, 
8. 
7, 
0, 
3, 
7, 
9.0 


4.0 


8.2 
3, 
6. 
0. 
0. 
) 
7. 
4, 
5. 


7.2 


7.1 


Null's. 


White:  blue. 


A  and  H.  } 
A  and  C.  f 


Yellow 
Yellow : 
Binary. 
Yellow  ; 


preenish. 
blue. 

blue. 


Biiiarv. 
A  and  B.  [ 
A  and  V.  \ 
Yellow  ;  blue. 

Yellow  :  blue  or  green. 

Yellow:  reddish  purple. 

Yellowish  :  l)luish. 

A  and  B.  }  Binary 

B  and  C.  \ 

Binary. 

A  ami  B.  )  Binary. 

A  and  C.  S 

Bed :  green. 

Yellow  :  emerald. 

Yellow :  purple.  Binary 


Golden  yellow :  blue. 
Light-green :  blue. 
Yellow :  blue. 

Yellow :  blue. 
A  and  B.  } 
A,  B,  and  C.  f 
Yellowish :  blue. 

Light-green :  blue. 
Yellow :  blue, 
yellow. 


White : 
Binary. 
Yellow 
White  : 


blue, 
blue. 


NoiK.— 'I  lie  sign  minus  (  —  )  befoie  decliuatiuns  meaus  south;  without  the  sign,  it  ia 
north. 


LIST  OF  NEIiULJi  AXD  STAR  CLUSTERS. 


525 


III. 


LIST  OF  THE  MORE  IXTERESTIXG  AXD  REMARKABLE  XEBULyE  AXD 

STAR  CLUSTERS. 


Object. 


47  Toucaiii  cluster 

(Jrcat  nebula  of  Andromeda . 
Nebula 

Tempcl's  variable  nebula. . . , 

Hind's  variable  nebula 

Globular  cluster 

Great  nebula  of  Orion 

Chacornac's  variable  nebula 
Nebula  around  i  Orionis .    . 

Looped  nebula 

Cluster  and  nebula  Mess.  40 

Star  cluster 

"         "       Mess.  67 

Planetary  nebula 

Nebula 

Planetary  nebula 

Spiral  nebula 

Nebula 

Star  cluster 

Bifid  nelmla 

Cluster  around  w  Centauri  . 

Spiral  or  v'm^  nebula 

Spiral  nebula 

Cluster 


R.  A, 1980. 

Due.  1h8(I. 

II.  M. 

O     1 

0  lit 

72  45  S. 

0  30 

40  37  X. 

0  42 

25  57  S. 

3  2',» 

30  32  S. 

3  3i) 

23  23  X. 

4  15 

19  14  X. 

5   <J 

08  55  S. 

5  10 

40  11  S. 

5  29 

5  29  S. 

5  3(» 

21   8  X. 

5  3(» 

1  17  S. 

5  oi) 

09  10  S. 

7  30 

14  32  S. 

7  48 

38  13  S. 

8  45 

12  15  X. 

0  11 

30  7  S. 

i»  18 

57  47  S. 

9  45 

09  38  X. 

10  2 

39  51  S. 

10  19 

18  2  S. 

11   8 

55  40  X. 

12  13 

15  5  N. 

12  17 

10  29  X. 

12  34 

10  57  S. 

12  30 

33  12  X. 

13  7 

18  48  X. 

13  18 

42  23  S. 

13  20 

40  41  S. 

13  25 

47  49  X. 

13  30 

29  10  S. 

13  32 

17  10  S. 

13  37 

28  59  X. 

526 


APPENDIX. 


Object. 


Cluster 

It 

Resolvable  nebula 

(Jreat  Cluster  of  Hercules 
Cluster 

n 
tl 
t( 

Small  annular  nebula. ... 

It  tt  ti 

Cluster 

Trifid  nebula 

Nebulous  cluster 

Hooked  nebula 

Cluster , 

Annular  nebula  of  Lyra. . . 

Variable  nebula 

Dumb-bell  nebula 

Small  annular  nebula 

Planetary  nebula 

Nebula  around  k  Cygni . . , 
Planetary  nebula 

Cluster 

tt 

Blue  planetary  nebula . . . , 


II.  A.  im>. 

Dec.  ISHO. 

H.  M. 

0 

t 

15  12 

2 

33  N. 

15  38 

37 

23  S. 

1«  10 

22 

41  S. 

IC)  37 

36 

42  N. 

1«  41 

1 

44  S. 

U   51 

3 

54  S. 

16  52 

44 

29  S. 

16  54 

29 

56  S. 

17  14 

38 

21  S. 

17  22 

23 

39  S. 

17  31 

3 

10  S. 

17  55 

23 

2  S. 

17  57 

24 

21  H. 

18  14 

16 

13  S. 

18  29 

24 

0  S. 

18  4'J 

32 

53  N. 

19   5 

0 

50  N. 

19  54 

22 

24  N. 

20  11 

30 

12  N. 

20  17 

19 

44  N. 

20  40 

30 

17  N. 

20  58 

11 

50  S. 

21  27 

1 

22  S. 

21  34 

23 

43  S. 

23  20 

41 

53  N. 

To  fucilitate  the  finding  of  the  above  nebula)  and  clusters,  their  posi- 
tions are  marked  ou  the  star-maps  with  small  circles. 


I'ERWDW  COMETS  SEEN  AT  MORE  THAN  ONE  RETURN. 


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530 


APPENDIX. 


VI. 


ELEM.EXTS   OF   THE   SMALL   PLANETS. 

COMl'ILKI)    ItY    I),  r.  TODD. 


Si(;n  iind  Niune. 

g  s 

Discoverer. 

a 

1  1 
Yrs. 

at 

-3  .2 

P 

c 

OS 

a 
c 

Mean 
Distance. 

// 

o 

o 

o 

(1)  Ceres 

ISOl 

Piazzl 

2.9S 

2.50 

770.2 

4.01 

0.077 

150.0 

80.8 

10.0 

2.709 

(2)  PnUas 

1S02 

Olbers... 

3.43 

2.11 

70S.  9 

4.02 

0.238 

122.0 

172.8 

34.7 

2.771 

(.=?)  Jmio 

1804 

Hardin;; 

3.35 

1.98 

S14.1 

4.30 

0.257 

54.9 

170.9 

13.0 

2.008 

(4)  Vcsia 

(5)  Astrsea 

1807 
1S45 

Olbers 

2.57 
3.00 

2.15 
2.10 

977.8 
850.9 

3.03 
4.14 

0.089 
0.180 

250.9 
134.9 

103.6 
141.5 

7.1 
5.3 

2.301 
2.579 

Ileutko 

(6)  Hebe 

1S47 

Hcucke 

2.92 

1.93 

939.9 

3.78 

0.203 

15.2 

138.7 

14.8 

2.424 

(T)  Iris 

(S)  Flora 

1S47 
1847 
184S 

Hind 

2.94 
2,55 
2.08 

1.83 
1.80 
2.09 

902.0 

10,S0.3 

902.3 

3.09 
3.27 
3.09 

0.231 
0.150 
0.123 

41.4 
32.9 
71.1 

259.8 

110.3 

08.5 

6.8 
6.9 
5.0 

2.3^0 
2.201 
2.387 

Hind 

Graham 

(9)  Metis 

(lO)IIygcia 

1849 

Gasparis 

3.49 

2.80 

030.4 

5.58 

0.109 

238.3 

285.5 

3.8 

3.144 

(11)  Partlieiiope  . 

1850 

Gasparis 

2.70 

2.21 

924.0 

3.84 

0.100 

317.9 

125.2 

4.0 

2.452 

(1-2)  Victoria 

(1!!)  Ef;oria 

1S50 
1^50 

Hind 

2.84 
2.80 

1.82 
2.35 

994.8 
857.9 

3.57 
4.14 

0.219 
0.087 

301.7 
120.2 

235.0 
43.2 

8.4 
10.5 

2.334 
2.577 

Gasparis 

(U)  Irene 

(15)  Euuoiuia  — 

1851 
1851 

Hind 

3.01 
3.14 

2.17 
2.15 

851.0 
825.4 

4.17 
4.30 

0.103 
0.187 

ISO.  3 
27.9 

SO.  8 
293.9 

9.1 
11.7 

2.691 
2.014 

Gasparis 

(10)  Psyclie 

1852 

Gasparis 

3.33 

2.52 

710.8 

4.99 

0.139 

15.1 

150.0 

3.1 

2.921 

(17)  Thetis 

OS)  Mi'lpomeue.. 
(19)  Fori  una 

1862 
1852 
1852 

Luther 

2.79 

2.80 
2.83 

2.15 
1.80 
2.05 

912.4 

1020.1 

930. 1 

3.S9 
3.48 

3.S2 

0.129 
0.21S 
0.159 

201.3 
15.1 
31.1 

125.4 
l.'jO.l 
211.5 

5.0 

10.2 

1.5 

2.473 
2.290 
2.442 

Hind 

Hind 

(20)  Massalia  .... 

1852 

Gasparis 

2.75 

2.00 

048.9 

3.74 

0.143 

99.1 

200.0 

0.7 

2.409 

021)  Lutctia 

1852 

Goldschmidt... 

2.  S3 

2.04 

933.0 

3.S0 

0.102 

327.1 

80.5 

3.1 

2.436 

(22)  Calliope 

(•Ja)  Thalia 

(24)  Themis 

1852 
1852 
1S5i! 

Hind      .  . 

3.20 

2.02 
2.02 
2.75 

715.2 
S32.4 
039.0 

4.90 
4.27 
5.50 

0.101 
0.231 
0.124 

69.9 

123.S 
144.1 

00.0 

07.7 
36.S 

13.7 
10.2 

0.8 

2.909 
2.029 
.3.130 

Hind 

3.24 

(fasparis 

3.52 

(25)  I'hociea 

185:! 

Cliai'oruac 

3.01 

1.79 

954.2 

3.72 

0.255 

302.8 

214.2 

21.0 

2.400 

(20)  I'rosorpinc.. 

(2V)  Eiiterp(! 

28)  Hcllona 

lS5a 

I85:i 

1854 

Luther 

2.89 
2.70 
3.20 

2.42 
1.94 
2.35 

819.7 
980.7 
700.0 

4.33 
3.00 
4.03 

0.087 
0.174 
0.153 

230.4 

88.0 

122.4 

45.9 

93.9 

144.7 

3.0 
1.0 
9.4 

2.050 
2.347 
2.777 

Hind 

Lutlior 

(29)  Ampliitrilc.. 

1S54 

Martb 

2.71 

2.34 

S09.0 

4.09 

0.074 

60.4 

350.7 

0.1 

2.526 

(:iO)  Urania 

1864 

Mind 

2.00 

2.00 

975.4 

3.04 

0.127 

32.1 

30S.1 

2.1 

2.3r,5 

ELEMEXTS  OF  THE  SMALL  PLANETS. 


531 


Si|;n  iiml  Niunc. 


(Hl)Eni)hrosync.  1S54 

(H2)  I'oiiioiia 1S54 

(iiit)  rolyliyninia.  1854 

(iU)  Circu iss.-) 

(3fi)  Leucothea. . .  ISSft 

(30)  Atalaiita  . . . 

(:iT)  l-'idfs 1855 

1S5G 
185(1 
1S5(> 


Uiscovvrtir. 


Fer};usou 

Goldtichinidt. . . 

CliacoiiKic 

t'liaconiac 


3.85 
2.  SO 
3.s;! 
2.'J7 
Luther i  H.CG 


»c 


(38)  Leda 

(;ut)  LaMitia.... 
(40)  llarmt)iiii'.. 


(41)  Daphne  . 

(4'2)  Isis 

(43)  Ariadne. 
(44)Nysi.... 
(45)  Edgenia. 


(40)  Hestia.... 
(4T)A^'laia.... 

(4s)  Doris 

(4H)  Pales 

(50)  Virginia,. 


(51)  Neniaiisa  — 

(52)  Euroiia 

(.-):()  Calypso 

(.54)  Alexandra.. 
(55)  Pandora 


(50)  Melete 

(57)  MncniDtiyiie. 
(5s)  Ooneordia. . . 

(.50)  Eliiis 

(60)  Echo 


(01)  Danao.... 

(02)  Erato  . . . . 

(03)  AilHonia.. 
((i4)  .\nt,'elina, 
(().5)  ('yl)ele  ... 


18.55    (ioldselunidt 

Luther 

Chacoriiac. . . 
Chacornac. . 
(ioldschniidt. 


1S50 

1 8.^)0 
1S57 
18.57 
1857 

1S57 
18.57 
18.57 
1S57 

1857 


Goldschniidt., 

l*()i;son 

I'oi^son 

(Joldschniidl. 
Goldsclimidt. 

PogHon 

Luther 

Ooldscliniiclt. 
Goldsehinidt. 
Ferguson 


(00)  Maia 

(07)  Asia 

(OH)  Lcto 

(00)  Hesperiu. 
(7  r)  Pauopuja. 


I 


(71)  N'iohe... 

(72)  Feronia. 

(73)  Clylia... 

(74)  Galatea.. 

(75)  Eurydice 


1858    Laurent  

ls,5S    Gdldschmidt. 

ls,58  !  Luther 

1S.5S  :  (ioldschniidt. 
1S58  \  Searle 

1S57    Goldschniidt. 

18.50    Luther 

ISOO  j  Luther  

isiio    ('hacornac. . . 
1^00    Ferguson.. . . 

18(;o  1  Ooldschinidt. 

18ti0  !  Fiierster 

lS(ii  j  Gasparis 

1^01     'I'enipel 

1801 

1801 
1801 
1801 
1801 
1801 

1801 
1801 
1802 
1802 
1801 


lenipel 

Tuttle 

PogBon , 

Luther 

Schiaparclli.. 
Cioldsi'lunidt. 


Luther.. . 
I'eters... 
Tuttle... 
Tenipcl.. 
Peters... 


?  3 


3.57 
3.11 
3.10 
3.(tS 
2.37 

3.51 
2.00 
2..57 
2.70 
2.04 


2.45 
2.37 
1.80 
2.40 
2.32 

1.02 
2.17 
2.32 
2.40 
2.10 

2.02 
1.80 
1.8;! 
2.00 
2.50 


""  13 

II 
035.3 
8.52.0 
733.3 

son.s 

085.0 


2.94    2.11 
3.25  ,  2..50 


3.33 
3.81 
3.41 

2.52 
3.35 
3.15 
3.25 
3.15 


Yra. 
5.5!' 
4.10 
4.84 
4.41 
5.18 


u 


0.223 
0.083 
0.340 
0.1(i7 
(»."24 


780.0    4..55    0..302 
82(5.4  I  4.30    (».177 


782.1    4.54 

7(!0.8  1  4.01 

1030.3    3.42 


773.3 
030.9 
los.5.0 
040.5 
791.0 

884.0 
725.0 
040.4 
055.3 
821.6 


2.80 
2.30 
l.'JO 

2.21 
2.70 

2.08  8;!0.5 
2.17  705.0 
2.37     774.0 


051 


4.50 
3.81 
3.27 
.3.78 
4.49 


O.IM 
0.111 
0.04  f 


l^ 


0 

93.4 
193.4 
342.4 
14S.7 
202.4 

42.0 

60.5 

101.2 

3.2 

0.0 


0.220    318.0 
0.107    278.0 


II 

3  '° 

a 
1 

3 

o 

O 

31.5 

20.5 

220.7 

5.5 

0.2 

1.9 

184.S 

5.4 

355.8 

S.2 

359.4 

IS.  7 

8.3 

3.1 

290.4 

7.0 

157.4 

10.4 

93.0 

4.3 

170.2 

10.0 

0.1.51 
O.0S2 


4.02    0.105 
4.80    0.130 


5.49 
5.42 
4.32 


97.5.4   3.64    0.067 


3.2'>  I  1.08 
3.50  !  '.'.81 
2.81  :  •2.,50 
3.03  I  2.40 
2.83  j  1.05 

3.47  j  2..50 
3.07  j  2.50 
2.09  j  2.10 
3.02  2.34 
3.80  ,  3.05 


3.00  2.21 
2.87  [  1.97 
3.30  2.20 
3.49  ,  2.47 
3.09  j  2.14 

3.23  2.28 

2..54  1.09 

2.78  2.55 

3.44  2.12 

3.49  1.85 


2  5.45 
4.25 
4.40 
4..50 


0.071 
0.2.35 
0.285 


112.2 
220.0 

3.54.2 

312.8 

70.3 

31.6 

10.1 

175.2 


3.148 
2.  .587 
2.80! 
2.080 
2.904 

2.745 
2.642 

2.740 
2.770 
2.267 


84.5      8.6  1  2.440 


204.0 
131.1 
148.2 

1S1.5 
4.3 
18.5.2 
200.7 
173.8 

175.9 


3.5  2.203 
3.7  2.423 
0.6  I  2.720 


2.3  ,  2..520 
5.0  I  2.>M) 
6.5'  ,3.112 


0.109  i  107.1  I  129.7 


0.204 
0.109 
0.142 


848.1 

4.19 

0.236  I 

633.  (( 

.5.01 

0.109 

790.0 

4.44 

0.042 

704.0 

4.47 

0.117 

058.3 

3.70 

0.184 

0S7.5 

.5.10 

0.102 

040.0 

5..54 

0.173 

955.6 

3.72 

0.124 

.S0S.3 

4.39 

0.128 

.V(8.9 

0.35 

0.110 

824.0 

4.32 

0.105 

041.5 

3.77 

0.186 

705.3 

4.04 

0.188 

0^9.0 

.5.15 

0.170 

839.0 

4.23 

0.1  S3 

775.4 

4.58 

0.173 

lOHM 

3.41 

0.120 

81.5.4 

4.35 

0.042 

705.0 

4.64 

0.238 

812.3 

4.  .37 

0.300 

93.0 

294.3 

12.1 

294.0 
54.1 

1S9.2 
18.4 
98.6 

344.1 

38.5 

270.4 

123.7 

200.8 

40.4 

:'.oo.4 
345.2 

108.5 
299.8 

221.3 

311S.0 

57.0 

8.0 

335.5 


144.0 

313.8 

10.9 

194.1 
200.2 
161.4 
170.4 
192.1 

334.2 
12.5.7 
338.0 
311.3 

1.58.8 

8.3 
202.8 

4.5.0 
1S7.2 

48.3 

31(i.5 
207.8 
7.9 
107.9 
3.50.0 


3.1 

3.084 

2.8 

2.652 

10.0 

2.3(i5 

7.4 

3.02(i 

5.1 

2.020 

11.8 

2.709 

7.2 

2.700 

S.O 

2.590 

15.2 

3.155 

fi.O 

2.700 

S.(i 

2.713 

8.0 

2.393 

18.2 

2.987 

2.2 

3.130 

5.S 

2.308 

113 

2.681 

3.5 

h.428 

.3.1 

2.650 

6.0 

2.422 

8.0 

2.781 

8.5 

2.980 

11.6 

2.014 

23.3 

2.7.56 

54 

2.2i;rt 

2.4 

2.0,5 

4.0 

2.7MI 

6.0 

2.0-2 

532 


APPENDIX. 


.Si;;ii  :'ml  Nainc. 


(76)  Pieiii 

(7T)  Fii-irii 

(IS)  Diana 

(7!»)  EiiryiiDine.. 

(80)  Sappho 

(81)  Terpsichore 

(82)  Alcinoiie  . .. 

(83)  Ueutrix 

(84)  Clio 

(85)  lo 

(86)  Semele 

(87)  Sylvia 

(88)  Thisbe 

(89)  Julia 

(90)  Autiope.... 

(91)  ^giim 

(92)  Uiidiiia 

(9;j)  Minerva.... 

(94)  Aurora 

(95)  Arethiisa... 

(90  iE-le 

(97)  ClDiho 

(98)  lanthc 

(99)  Dike 

(100)  Uekate 

(101)  Helena 

(102)  Miriam 

(103)  Hera 

(104)  Clymene  . . . 

(105)  Artemis.... 

(lOC)Dione 

(107)  Camilla.... 

(lOS)  Heouba 

(lOd)  Felicitas  . . . 

(110)  Lydia 

(111)  Ate 

(112)  Iphi-renia  .. 
(111!)  Amalthea.. 

(114)  Casi-andra.. 

(115)  Thy ra 

(llG)Sirona 

(117)  Lomia 

(118)  Peitho 

(119)  Allhiea 

'20)  Laehe-sis.... 


>rr   C 


1802 
1S02 
1803 
1SC3 
1804 

1864 
1804 
1S05 
1865 
1865 

ISOO 
ISOO 
1800 
1S60 
1860 

1SC6 
1807 
1867 
1807 
1867 

1868 
1868 
1808 
1868 
1868 

1868 
1808 
1808 
1868 
1808 

1808 
1863 
1SC9 
lv809 
1870 

1870 
1870 
1871 
1871 
1371 

1871 
1871 
1872 
1872 
1872 


Oisicj 


D'Arrcst. 
Peters. . . 
Luther  . . 
Watson  . 
Pogsoii.. 


Tenipel.. 
Luther  . . 
Gasparis. 
Luther... 
Peters . . . 


Tietjen . 
Pogsou . 
Peters . . , 
Stephau  . 
Luther . . 


Stephan , 
Peters. . , 
Watson., 
Watson . . 
Luther... 


Coggia . 
Tenipel. 
Peters . . 
Borelly. 
Watsou. 

Watson , 
Petert . . 
Watson. 
Watson. 
Watson. 


Watson. 
Pogsou. 
Luther  . 
Peters. . 
Borelly . 


I'eters. . 
Peters. . 
Luther  . 
Peters. . 
Watsou. 


Peters. . 
Borelly. 
Luther  . 
Watson , 
Borcllv. 


^  c 


4.00 
.S.03 
3.10 
2.92 
2.76 

3.45 
3.38 
2.64 
2.92 
3.10 

3.76 
3.70 
3.21 
3.01 
3.68 


2.82 
2.31 
2.0>; 
1.97 
1.84 

2.25 
2.15 
2.22 
1.80 
2.15 

2.46 
3.21 
2.32 
2.09 
2.61 


2.87    2.31 
3.51  [  2.86 
2.37 
2.S9 
2.63 


.3.14 
3.44 
3.52 


3.48 
.3.36 
:'.20 
3.40 


2.62 
1.98 
2.18 
2.13 


3.00  :  2.58 

2.04  2.23 
3.47  I  1.86 
2.92  :  2.4S 
3.70  :  2.00 
2.79    1.96 


3.73 
4.00 
3.54 
3.5(( 
2.94 


2.59 
3.12 
2.88 
1.89 
2.52 


2.86  2.32 
2.74  I  2.12 
2.58  [2.17 
3.05  2.30 
2.84    1.92 

3.10  2.37 
.3.00  2.92 
2.83  [  2.05 
2.79  ,  2.36 


3.' 


2.97 


^1 

'J 

'■7  i 

Vr». 

II 

563.7 

6.30 

812.2 

4.37 

835.3 

4.25 

928.9 

3.82 

1019.8 

3.48 

736.2 

4.82 

771.4 

4.60 

930.7 

3.79 

970.9 

3.03 

820.7 

4.33 

040.3 

5.49 

MO.O 

6.50 

770.2 

4.61 

870.8 

4.  OS 

030.2 

5.58 

851.8 

4.17 

023.7 

6.69 

776.5 

4.57 

630.7 

5.03 

657.7 

5.40 

000.2 

5.33 

814.2 

4.30 

804.  S 

4.41 

75S.7 

4.08 

652.6 

5.44 

864.2 

4.16 

817.0 

4.35 

799.1 

4.44 

635.0 

5.59 

970.1 

3.60 

6.31.0 

6.62 

528.2 

6.72 

616.4 

6.70 

8sv2.0 

4.43 

786.4 

4.52 

849.9 

4.18 

9.34.7 

3.80 

908.8 

8.60 

S10.6 

4.38 

966.9 

3.67 

770.9 

4.60 

086.0 

6.  IS 

931.9 

3.S1 

«)5,0 

4.15 

643.5 

5.52 

0.174 
0.134 
0.205 
0.194 
0.200 

0.211 
0.221 
0.086 
0.236 
0.191 

0.210 
0.079 
0.160 
0.180 
0.109 

0.108 
0.102 
0.140 
0.080 
0.144 

0.140 
0.258 
0.189 
0.238 
0.164 

0.138 
0.303 
0.080 
0.174 
0.175 

0.181 
0.123 
0.103 
0.300 
0.077 

0.105 
0.1 28 
0.087 
0.140 
0.194 

0,143 
0.023 
0.161 
0.083 
0.047 


™  .2 

2^ 

SI 

I'i 

o 

o 

92.8 

212.2 

60.4 

2.0 

121.3 

334.1 

44.4 

206.7 

355.3 

218.7 

48.7 

2.7 

132.4 

27.0 

191.8 

27.6 

339.3 

327.5 

.322.6 

203.9 

29.7 

88.1 

335.4 

76.1 

309.3 

277.0 

353.4 

811.7 

301.1 

71.4 

80.3 

11.1 

330.8 

102.9 

274.7 

5.1 

46.0 

4.6 

31.2 

244.3 

103.2 

3-22.8 

65.6 

160.7 

14t.o 

354.4 

240.0 

41.7 

307.7 

128.2 

327.4 

.343.7 

354.0 

212.0 

321.0 

130.8 

58.2 

44.0 

242.8 

183.0 

27.0 

63.4 

112.8 

176.7 

173.5 

352.4 

50.0 

4.9 

336.8 

57.2 

108.7 

306.2 

338.2 

324.0 

198.7 

123.2 

153.1 

164.4 

43.0 

309.1 

152.8 

04.4 

4S.8 

349.0 

77.0 

47.6 

12.4 

204.0 

214.0 

342.9 

o 

2.0 

2.5 

8.6 

4.0 

8.6 

7.9 
2.9 
5.0 
9.4 
11.9 

4.8 
10.9 

5.2 
16.2 

2.3 

2.1 

9.9 

8.6 

8.1 

12.9 

16.1 
11.8 
16.6 
13.9 
6.4 

10.2 
6.1 
6.4 
2.9 

21.5 

4.0 
9.8 
4.4 
8.0 
6.0 

4.9 
2.0 
6.0 
4.9 
11.6 

3.6 
15.0 

7.S 
6.S 
7.0 


3.409 
2.072 
2.623 
2.444 
2.296 

2.8.53 
2.706 
2.430 
2.303 
2.654 

3.112 
3.482 
2.769 
2.551 
3.145 

2.689 
.S.187 
2.754 
3.103 
3.076 

3.050 
2.0)13 
2.689 
2.797 
3.092 

2.584 
2.662 
2.701 
3.149 
2.374 

3.160 
3.660 
3.212 
2.095 
2.733 

2.693 
2.433 
2.370 
2.676 
2.379 

2.707 
2.991 
2.438 
2.530 
3.121 


ELEMENTS  OF  THE  SMALL    PLANETS. 


Sit,'n  and  Name. 


(121)  Ilcnnioue 

(122)  Gerda .... 
(12:?)  Rriinhilda 
(124)  AlccHtc... 
(126)  Llberatilx 

(I'.'C)  VcUoda. . . 
(12T)  Joliaiiua  . 
(128)  NeincBis.. 
(121))  Antiijoiie. 
(130)  Electi-a 

(I.^l)  Vala 

(i:i2)  yEthra 

(13.^)  Cyieiie 

(134)  Rophi'o.syne. 
(13.5)  riurlha 

(136)  Austria 

(13T)  Melibcea.. .. 

(138)  Tolosa 

(1.39)  Jiicwa 

(140)  Siwa 

(141)  Luineu 

(142)  Polana 

(143)  Adria 

(144)  Vibilia 

(145)  Aduona 

(14(5)  Liiciiia 

(147)  I'r()tii;;eiieia 

(148)  (Jallia 

(14!))  Medusa 

(150)  Nuwa 

(151)  Abnndantia. 

(152)  Atala 

(153)  Hilda 

(154)  H(!rtba 

(155)  Scylia 

(1.5(>)  Xantli)i)C... 
(157)  Dcjaiiiia ... 
(15S)  Koroiiis.... 

(l.MO  /Eniilia 

(10(1)  Una 

(IC.l)  Albor 

(I(i2)  I.nurc'dtia  .. 

(1()3)  Eriiioiio 

(1(U)  Eva 

(^1(15)  Loreloy 


Discoverer. 


1S72  Watson.. 

1S72  Peter 

1872  Peters 

1S72  PcterB 

1S72  Pnispcr  Henry. 


1S72 
1S72 
1S72 
1S73 

1S73 

1873 
1873 
1873 
1873 

1S74 


Paul  Henry 

Prosper  Henry. 

Watson 

Peters 

Peters 


Peters.. 
Watson. 
Watson. 
Luther  . 
Peters.. 


1874  Palisa... 
1874  j  Palisa... 
1874  \  Perrotin. 
1874  Watson.. 
1874    Palisa... 


1875 
1875 
1875 
1875 
1875 

1875 
1S75 
1875 
1S75 
1876 

1875 
1875 
1875 
ls75 


Paul  Henry. 

Palisa 

Palisa 

Peters 

Peters 


Borelly 

Sebulhof. 

Prosper  Henry, 

Perrotin 

Watson 


Palisa 

Paul  Henry 

Palisa 

Prosper  Henry. 


-  3 


3.88 
3.34 
3.00 
2.84 
4.09 


c  - 


3.03 
SAO 
2.3S 
2.42 
1.9S 


2.70  i  2.1s 
2.92  i  2..5y 
3.10  I  2.40 
3.47  2.28 
3.77    2.47 


2.62 
3.59 
3.48 
2.S7 
2.93 

2.48 
3.78 
2.83 
2.96 
3.32 


2.22 
1.61 
2.64 
2.27 
1.93 

2.09 
2.48 
2.06 
2.67 
2.14 


3.31    2.10 
2.64    2.14 


1S75    Palisa. 


Palisa 

B<n-elly 

Knorre 

Paul  Henry. 
Peters 


1875 
1875 
1876 
ls76 
ls76 

1876 

1876 

1876 

1876  j  Paul  Henry, 

ls76  I  Peters... 


Watson 

Prosper  Henry. 
I'errotin 


2.93 
3.27 
3.27 

2.89 
3.22 
3.28 
2.39 
3.38 


2.S4 
3.i!9 
4.00 
3.54 


2.  ,57 
2.03 
2.12 


2.53 
3.03 
2.26 


552.5 
614.1 


Vrs. 
6.43 
5.78 


803.4  j  4.42 
832.0  '  4.27 
671.0   5.29 

931.0  13.S1 
775.9  j  4.58 

777.5  (  4.57 
727.4  ,  4.88 
643.9    5.51 


0.122 
0.037 
0.115 
0.073 
0.347 

0.106 


1.3 

208.9 

72.9 

24!5.7 

261.3 

347.8 


942.8 
845. 1 
662.2 
864.0 
937.1 

1025.9 
639.7 
928.8 
751.0 
785  9 


3.77 
4,20 
ii.36 
4.12 
3.79 

3.46 
5.55 
3.82 
4.72 
4.52 


962.0  3.60 
777.0  4..57 
S22.4 

S(r.'.5 


796.3 
(542.2 
769.8 
1.88|1139.2 
2..59    685.2 


2.33 
2.87 
3.31 
2.90 


3.84  2.24 
3.16  2.02 
3.86  2.12 
3.49  I  2.76 
2.90    2.57 


2.(59 
3.54 
2.71 
3.37 
3.30 


2.00 
2.52 
2.00 
1.73 
2.90 


854.2 
640.1 
451.9 
613.S 


4.46 

.5..53 
4.61 
3.12 


4.16 
5.55 
7.S0 
5.7S 


070.2  5.30 

S53.4  16 

08(i.2  ,">.17 

642.2  ,«  .53 

783.6  4.53 

90'<.8  !  3.60 


(573.1 
982.1 
870.1 


.5.28 
3.02 
4.08 


0.000 

122.6 

0.126 

16.6 

0.207 

241.0 

0.208 

20.5 

0.082  258.6 
0.380  152.4 
0.137  ;  248.3 
0.117  ;    66.9 


1 

a 

Mean 
Distance. 

0 

T.6 

3.455 

1.0 

3.22(1 

0.5 

2.(592  ; 

2.9 

2.030 

6.1 

3.035 

2.9 

2.440 

8.3 

2.755 

0.3 

2.751 

12.2 

2.«70 

0.205 

0.085 
0.20s 
0.158 
0.061 
0.210 


I 


95.0   4.40    0.223 
0.105 
0.(160 
4.32    0.233 
4.42    0.213 


0.007 
0.030 
0.185 
0.119 


5.18    0.132 


641.2  :  5.54 


0.100 
n.0S2 
0.163 
0.100 


'^.264 
0.220 
0.292 
0.110 
0.001 

0.133 
0.169 
0.149 
0.321 
0.073 


319.8 

310.5 
3111.3 
311.4 
11.5.5 
300.0 

22.0 

227.4 

V23,3 

8,3 

118.1 

237.7 

84.7 
30.0 

.3.57.5 

215.9 

80.0 

2S5.0 

108.7 


1.5,5.0 
109.2 
3,55.2 
1(10.7 
50.8 

312.9 

143.0 

93.3 

2.8 


146.0 

22.9 

0,5.3 

4.0 

200.0 

25.0 

321.2 

7.2 

346.5 

11.6 

344.0 

2.3 

180.2 

0.0 

204.3 

13.8 

54.9 

3.2 

107.0     3.2 


319.1 

292.6 

333.  S 

76.8 

77.7 

84.4 
2,52,5 
145.1 
160.1 
207.5 

■10,0 
41.5 

228.3 
37,6 


246.2 
02.4 

282.8 

135.1 

9.8 

18.5 

3S.2 

158.8 

77.4 


11.5 
2.3 

11.5 
4.9 

14.4 


3.120 

2.419 
2.603 
3.062 
2.567 
2.429 

2.2S7 
3.133 
2.444 
2.814 
2.732 

2.709 

2.387 
2.7,52  i 
2.650  j 
2.604 


12.7    2.7  i> 
2.0    3.1-.'5 


25.3 
1.1 
2.2 

7.9 
12.2 

7.8 
20.8 


7.6 
11.8 
1.4 
6.1 
8.8 

9.2 
6.3 
4.7 

>4.8 


2.770 
2.132 
2.981 

2..584 
3.132 
3.954 
3.221 


3.037 
2.586 
2.990 
3.125 
2.737 

2,K7(! 
3.0"9 
9.3M 
2.5.53 


.304.0  I  11.2    3.129 


53-i 


APPEXniX. 


Si^^  find  Noine. 

^      -X- 

>•  .'£ 

c 

1870 
IS'C 
1S70 
1870 
1877 

1877 

1S77 

Discoverer. 

il 

O 

?    S 

Yra. 
4.49 

5.78 
0.21 
ii.02 
4.0S 

5.5S 
3.08 

"3   e 

—  .2 

o 
32.7 

1*       . 

a 
o 

c: 

.5 

B 
O 

11.7 
1.7 
4.0 
5.5 

14.3 

2.0 
10.0 

9J 

(IGO)  Rliodope . . . 

(107)  Urdu 

(lOs)  JSihylla 

(lfi!»)  Zeliu 

(170) 

U71) 
(17-2) 

PetcrB 

ii.37 

2.07 
2.22 
if.  15 
2.05 
2.38 

2.70 
2.11 

II 
701.0 
014.5 
571.5 
'.)7!».9 
870.  S 

o:?o.s 

!)05.!> 

0.239 
0.312 
0.007 
0.131 
0.0C5 

0.121 
0.113 

o 
129.2 
170.1 
209.0 
.3.54.0 
301.3 

101.9 
331.8 

2.720 
3.218 
3.378 
2.358 
2.551 

.3.143 
2..381 

Peters 

4.'.''2 
a.OI) 
'2.07 
2.72 

:i.f)2 
2.05 

Wiitsioii 

ProsiKU-  Ilcnry. 
Perrotin 

Borolly 

IJorelly 

REMARKS  ON   THE  PRECEDING  ELEMENTS  OF  THE  PLANETS. 


MasHcu. — Tho  iiuisscs  of  iimny  of  tlio  planets  arc  still  voiy  uncertain, 
because  exact  oh.servalioMs  luive  not  yet  been  made  loiijj;  enough  to  per- 
mit of  their  sati.slactoiy  detennination.  The  mass  of  Mercury  may  be 
estimated  as  uncertain  by  ,^  of  its  entire  amount;  that  of  Slars  by  ^.^', 
those  of  Venus,  the  earth,  Urainis,  and  Neptune  by  iJg  ;  while  thojso  of 
.'npiter  and  Saturn  are  iirol>ai>ly  correct  to  lu^yg. 

The  value  of  tho  earth's  mass  which  we  have  ftiveu  does  not  include 
that  of  the  moon.  The  mass  of  the  latter  is  estimated  at  rixhz  that  of 
the  earth. 

Tho  ma.sscs  of  Jupiter,  Saturn,  Uranus,  and  Neptune  which  we  have 
cited  are  all  derived  from  observations  of  the  satellites  of  these  planets. 
The  masses  derived  from  the  ]>crturbations  of  the  planets  do  not  differ 
from  them  by  amounts  exccediiifj;  the  uncertainty  of  the  determinations. 
The  mo.st  noteworthy  deviation  is  in  the  case  of  .Saturn,  of  which  Le- 
verrier  has  found  the  mass  to  be  rfs^.yrjy,  a  result  entirely  incompatible 
with  the  observations  of  the  satellites. 

Didincftri^. — These  are  also  uncertain  in  many  cases,  especially  in  those 
of  the  outt^r  i)lanets,  Uranus  and  Neptune.  Th(!  densities  which  we  have 
assigned  to  these  last-mentioned  planets,  depending  on  their  masses  and 
diaiiu'ters,  must  be  regarded  as  uncertain  by  half  t'seir  entire  amounts. 

EJlipt'ic  AVrmc/i/.v.  -Of  these  it  may  be  .said  that  in  gcucr.'il  they  are  very 
accurate  for  the  planets  nearest  tli(>  sun,  ))nt  diniiui>li  in  ))iecision  as  wo 
go  outward,  those  of  Neptune  being  d(>ul)tfnl  by  one  or  more  minutes. 

EkmcutH  of  thf  Snuill  J'htuclx. — These;  are  only  giveu  approximately,  iu 
order  that  tht!  reader  may  see  the  relations  >''  the  group  at  a  glance. 
They  are  mostly  taken  from  thi^  Jitrliiitr  .lutroiHWihclirn  .hdnhitrh.  \\\\\A\ 
gives  annually  the  latest  elements  known.  The  elements  of  the  twenty 
or  thirty  last  ones  ans  very  uncertain. 


DETEliMlXATIONS  OF  STELLAR  PAIiALLAX, 


535 


VII. 


DETERMIXATIOXS  OF   STELLAR  PARALLAX. 

The  following  i.s  .1  list  of  tlic  stars  the  \,..  allaxos  of  whicli  aro  known 
to  be  investigated,  with  the  results  obtained  by  the  different  investiga- 
tors. The  years  are  generally  those  in  which  the  observations  aro  sup- 
posed to  have  been  made,  1>nt  in  (he  case  of  one  or  two  of  the  earlier 
determinations  they  may  bo  those  of  the  ptiblication  of  results.  In  tho 
references  the  following  abbreviations  are  used: 

A.  G.  riihJiral'hnir)!  iUr  AMronomhchcn  OcscUschuft. 

A.  N.  ^Istrtnioinisrhf  Xdcliriclilm. 

15.  M.  Moiiatubirirht  (of  the  Tu'rlin  Academy  of  Sciences). 

C.  R.  Cimpkn  Ri)i(b(H  (of  the  French  A(.'adeniy  of  Sciences). 

D.  O.  Antroiiomiial  Ohncrratioii,  etc.,  at  Dniixiii'ih-,  bj  Francis  Uriinnow. 

2  Farts.     Dublin,  ]s70  and  ld74. 
Mel.  Mi'htiificn  MathaiuitiqiK'H  it  AttlniiKiDiiqiioi,  .icitdanic  dc  SI.  Pclem- 

hoitrt/. 
M.N.  JUuiitlilii  Xoticcn  of  the  Ilojittl  A^lroiioinical  Sociilij. 
M.  R.  A.  S.  Memoirs  of  the     niidl  A^^lrouotnirol  Societi/. 
M.  F.  Memoiirs  de  /'.//•<■  ■  '.nte  de  Seinice-i  de  >7.  Pelevfilnmyij. 
F.  M.  lieeiieil  des  Meiiioireii  den  ^lntroiit)ine>i  de  I'oulkowa,  puhlie  pur  W, 

SIruve.     St.  Fetersbourg,  1^3:?,  vol.  i. 
R.  O.  liadeliffe  Observations,  Oxford. 


star's  Niiriu' 


Groonil)!- 

No.  ;u. 

Pole  Star 


[ 


('ill)Cllll... 

Sii'iim 


AstriiuonuT,  anil  UiUv. 


( Atnvei's,  from  heliomoter  nicasurcsi, ) 

"(     18(i:i-'(i(i ) 

LiiuloiiMli,  from  H.  A.'s,  IT.'iO-lSlO 

W.  Striivi",  Doriint,  1S1S-'21 

Stniveiuul  Prciiss.from  U.  A.V,1822-'HS 
Liiiulalil.  from  Dorpiit  dt'cliii.-itioiis. .. 

Totors,  from  tlocliiKilioii.^,  lS42-'44 

binaiiML'cn 

I'ottMs,  from  docliiKitions,  1842 

Striivc,  with  Pulkowi!  f  lUiU.,  Isri5  — 
IlcMulcri?on,  is;!;i 


Piirnlltix. 

rr..l.:il.l.> 

Ul-r.TUlML'. 

0.292 

±  .():!G 

li.  M..isr,7. 

0.144 

P.  M.,  p.  05. 

O.OT.') 

0.1  T2 

0.14T 

±  .o.-io 

0.(M)7 

P.  M.,1..  121. 

0.02.'> 

±.01S 

P.  M.,i).  2fi4. 

0.04(i 

±.20 

P.M.,  p.  136. 

o.-m,') 

±.04;i 

Md.,  II.,  p.  400. 

o.;u 

P.M..  p.  04. 

530 


APPENDIX. 


Star'ft  Name. 


Sirius. 


Castor 

1  Vrssc  Maj 

Lalaiule    No.  [ 

'2US5 ) 

Lalaude    No. ) 

21258 i 

Groonibridge  ) 
No.  1830  . . .  [ 


Oeltzen    Arg.  ^ 
N.,  No.  17415) 

/J  Cciitanri 

a  Bootls 


a  Centauri . 


p  Ophiuchi. 


Astronomer,  and  Date. 


Maclear,  1 S37 

( Ilcnclersoii,  from  his  own  and  Mac-  [ 

(     lear's  observations ) 

Gyldcu,  from  IMaclear's  obs.,  1830-37. 

Al)be,  from  Cape  obs.,  1S.5C-'C3 

(Jolmson,  witli  Oxford  lieliometer,  | 

\     lS54-'55 f 

Peters,  from  declinations,  1842 

Winnecke,  with  heliometer,  1857-'CS. . 

Auwers,  1860-'C2 

Krueger,  1SC2  (?) 

Peters,  from  declinations,  1S42 

Faye,  at  the  Paris  Observatory 

( Wichman,  from  Schliiter's  observa- ) 
\     tions,  lS42-"43 ) 

Wichmaun,  from  his  own  obs.,lS51t-j 

Struvc,  1847-'49 

Johnson,  with  heliometpr,  1854-'55.. . . 

Auwers,  from  Johnson's  obs 

Briinuow,  1S70-'71 

Krueger,  1SG2  (?) 

Moesta,  from  declinations,  1800-'64 . . . 
Peters,  from  declinations,  1842 

Johnson,  Oxford  heliometer,  1845-55. 

(Henderson,  from  his  meridian  obs. ) 
'(  at  the  Cape  of  Good  Hope,  1832-'33. ) 
a'  Centaiu-i,  from  right  ascensions  . .. 
a'  Ccntaui-i,  from  direct  declinations. 

a'  Centauri,  from  reflected  decs 

a^  Centauri,  from  right  ascensions  ... . 
ii"  Centauri,  from  direct  declinations. 

u^  Centauri,  from  reflected  decs 

Mean  of  jill  for  botli  stars 

Peters,  from  the  same  obs.,  finds 

(  Henderson,  from  Maclear's  observa- ) 

'(     tions,  lS3!l-'40 [ 

Peters,  from  the  same  observations  .. 
Maclear.  from  decs.,  lS42-'44  ar.d  1848. 
iSIoesta.  from  declinations,  lSG0-'64 . . . 
Krueger,  1858-59 


raralliix. 

Probiililu 
Error. 

ff 

„ 

0.10 

0.23 

.... 

0.193 

±.087 

0.273 

±.102 

0.210 

±.062 

0.133 

±.100 

0.501 

±.011 

0.271 

±.011 

0.200 

±.020 

0.220 

±.141 

1.08» 

.... 

O.ISO 

±.018 

0.085 

±.018 

0.080 

±  .023 

0.034 

±.029 

0.033 

±.028 

0.023 

±.033 

0.09 

±.01 

0.247 

±.021 

0.213 

±.009 

0.127 

±.073 

0.138 

±.052 

0.02 

±.35 

1.42 

±.19 

1.90 

±.47 

0.48 

±.34 

1.05 

±.18 

1.21 

±.04 

1.10 

±.11 

1.14 

±.11 

0.013 

0.970 

±.004 

0.919 

±.034 

0.880 

±.068 

0.109 

±.010 

Uefi-rcncc. 


M.R.A.S.,xi.,248. 

Mel.,  in.,  595. 
M.N.,xxviii.,  p. 2. 

R.O.,xvi.,  p.  (xl). 

P.M.,  p.  136. 

A.  O.,  No.  xi. 

A.  N.,  No.  1411. 
M.N.,  xxiii.,173. 
P.  M.,  p.  136. 

C.  R.,  xxiii. 
A.N., vol. 30,  p. 29. 

[/6.,p.33, 

P.M., p.  291. 
JR.  O.,    xvi.,    p. 
(     (xxii). 
13.  M.,  1874. 

D.  O.,  11.,  p.  23. 

M.  N.,  xxiii.,  173. 

A.  N.,  16SS. 
P.M.,  p.  130. 
(R.  O.,    xvi.,    p. 
]     (xxiii). 


M.  R.  A.  S.,  xi., 
p.  67-68. 

J 

P.  M.,  p.  62. 

(M.  R.  A.  S.,  xli., 

(     ]).  370. 

P.M.,  p.  63. 

M.R.A.S.,xx.,9S. 

A.  N.,  loss. 

A.  N.,  1212. 


•  This  resnlt  is  probably  erroneous. 

t  These  results  of 'Wichniann  are  i)arallaxes  relative  to  the  mean  of  certain  stars  of  com- 
parison. He  concluded  that  one  of  the  latter  lia<l  a  largo  parallax  which  made  the  paral- 
lax of  1830  Or.  0".72  ;  but  this  view  was  afterwards  proved  wrong. 


DETERMiyAXlOXS   OF  STELLAR  PARALLAX. 


537 


Star'*  Name. 


p  Ophiuchi , 
a  Lyra; 


a  Cygni . . 
eiCygui. 


Aatr'iiiomi'r,  jiiul  Pi»to. 


Krueger,  18fiS-'()2 

Airy,  Troughton's  circle,  1836 

Airy,  Joucs'h  circle,  1830 

Striivc,  1837-4(1 

Petcrn,  from  (Iccliiiatioim,  1S4'2 

Slruvf,  issi-'ns 

J(>hii(<(>n,  1854-'55 

Briimiow,  18(iS-'Gi> 

Briilinow,  1870 

Peters,  fniin  decliiKitioiis,  18-12 

(Bessel,  with  Kdnigsl)erg  heliome-) 

I     ter,  1838 ) 

Bessel,  from  siiibxcqiient  obs.,  1840 

Peters,  from  (Iccliiiiilious,  1842 

(Johnson,  with  Oxford  heliometcr, ) 

(     lsr)2-'53 )' 

Auwers,  from  Johnson's  obs 

Striive,  l<52-'53 

Auvvers,  from  Konigsberg  heliometer, 


rariilhijc. 


0.162 
0.224 

-0.102 
0.202 
0.103 
0.147 
0.154 
0.212 
0.1 8S 

-0.082 

0.314 

o.:u8 

0.349 

0.392 

0.42 

0.500 

0.504 


l'r,.biil,l,. 
Krror. 


±  .007 


±  .053 
±.009 
±.040 
±.010 
±  .033 
±  .043 


±  .080 


±.028 

±.010 


Rtifurence. 


A.  N.,  1403. 
\  M.  R.  A.  S.,  X., 
(      p.  209-270. 
P.  M.,  p.  5s. 
P.  M.,  p.  130. 
M.  P.,vii.,vol.  i. 


I).  ().,  T'art  I. 
I).  O.,  Piut  II. 
1'  M.,  p.  130. 


P.M.,  p.  130. 
U.  0.,vol.  xiv. 


M.  P.,  VII.,  I.,  45. 

A.N.,14n-'10. 


538  AVVENDIX. 


VIII. 

SYNOPSIS  OF  PAPERS   OX   THE   SOLAR  PARALLAX,  1854-'7'7. 

Tbo  following  is  believed  to  bo  a  nearlj'  complete  list  of  the  tlotermi- 
iiiitions  of  tbe  solar  parallax  wbicb  have  appeared  Mince  the  discovery  of 
tbe  error  of  tbe  old  parallax  in  1H54.  No  papers  have  been  inclnded  ex- 
cept those  which  relate  immediately  to  the  determination  in  question. 

1.  IIaxskn,  1854— il/.TV^.  li.  A.  S.,  xv.,  p.  9. 
Statement  that  be  finds  the  coefticient  of  tlus  ])arallactic  Cipiation  of  the 
moon  to  be  Vi.W.'li)') — a  value  <j;reater  than  tliat  deduced  from  the  solar 
]tarallax  as  given  by  tbe  transits  of  Venus. 

2.  LKVKRmF.ii,  1858 — Amiahn  <iv  V Ohscrvatolre  <lc  rnr'iK,  iv.,  p.  101. 
Discussion  of  solar  i)arallax  from  lunar  equation  of  the  earth,  giving 
8".95.     (In  this  ])a])or  Mr.  Stone  has  found  two  small  numerical  errors: 
correcting  them,  there  results  8".85.     There   is  also  a  doubt  about  the 
theory,  which  might  allow  the  result  8". 78.) 

3.  FoccAri.T,  18(52 — Compics  Itvudus,  Iv.,  ]>.  .501. 

Experimental  determination  of  the  velocity  of  light,  leading  to  the  value 
of  tbe  st)lar  jtarallax,  8".86. 

4.  llALi,,  If^G'A—Witxhinofoi)  OhscrratioiiH  for  18(;3,  p.  Ix. 
Solar  parallax,  deduced  from  observations  of  Mars  with  equatorial  in- 
struments, in  1862 :  result,  8".8415. 

5.  FKKiiUsoN,  18(5;? — WasluiujUm  Ohscrral  inns  for  ]8(5:?,  p.  Ixv. 

Solar  parallax,  deduced  from  observations  with  meridian  instruments  at 
Washington,  Albany,  and  Santiago.  Kesults  various  and  discordant,  ow- 
ing to  iucomplctcness  of  the  work. 

G.  Stoxk,  18G3— 3/.  ^V.  />'. ./.  ,S.,  xxiii.,  p.  183  ;  Mem.  li.  A.  8.,  xxxiii.,  p.  97. 
Discussion  of  fifty-eight  corresponding  observations  of  Mars  (tweuty-ouo 
pairs)  at  Greenwich,  Cape,  and  Williamstowu,  leading  to  8".U43. 


SiWol'Sis   OF  I'AI'EUS   ON  SOLAIi  PARALLAX,  1854-77.   539 

7.  Hanskn,  18ra— .1/.  .V.  /.'.  A.  S.,  xxiii..  ]..  2J:?. 
Deduction  oftlic,  v;iliu' H '.!)7  from  lli»!  iiiiuilhictic  iiu-qiiiility  oftlio  moon. 

H.  1  lANsi-.N,  18g:j— .1/.  .v.  n.  a.  s.,  wW.,  p.  h. 

A  more  accurate  computation  IVom  thf  .same  data  gives  H".9l.")'J. 

9.  WiNXKCKK,  lH(;:i  -J,s/,-.  y<(rl,r.,\\\.,  col.  '^t')!. 

Comimrison  ottwcnty-six  coricsitondin^  oliscivatinn.s  (thirteen  pairs)  at 
I'ullvowa  and  tlie  Capo  of  (Jood  Hope.     J'arallax,  IS".1)()4. 

10.  ToWAi-KY,  181)4 — Doctoral  DisHortatioii,  translated  in  Coiinuixmncc  (Ich 

Tcmpx,  18()7. 
Discussion  of  the  transit  of  Venus,  17('i!».     Result,  8".832,  or  8".8()  when 
tlie  longitude  of  Chappe's  still  ion  is  left  arbitrary. 

11.  Stonk,  1807— .V.  X  /;../.  .S'.,  xxvii.,  p.  y:?(». 

Attention  directed  to  a  sliglit  lacl<  of  precision  in  Hansen's  first  paper 
(No.  7).  Deduction  also  from  its  data  of  the  result  8  '.'Jlti — agreeing  with 
that  from  Hansen's  secon<l  paper. 

12.  Stonk,  1807— J/.  X.  n.  A.  S.,  xxvii.,  p.  241. 
Correction  of  one  of  the  nnmerical  errors  in  Leverrier's  determination. 

liesult,  8".91. 

13.  Stonk,  1867— .V.  X.  /.'.  A.  s.,  xxvii.,  p.  '>71. 
Determination  of  the  parallactic  ine(iuality  of  tlio  moon  from  207.')  ob- 
servations at  Grcenwieli.     Inecpiality,  12r)".'.?().     Solar  parallax,  8".85. 

14.  Nkwc'omij,  1807 — Wafihinriton  Ohscrrations,  180.5,  Appendix  11. 
Discussion  of  the  principal  methods  employed  in  detennining  the  solar 
parallax,  and  of  all  the  meridian  observations  of  Mars  during  the  opposi- 
tion of  18G2.     Result,  H".848. 

1.5.  S  TONE,  18C7— .1/.  X  1{.  A.  S.,  xxviii.,  p.  21. 
Comparison  of  Newcomb's  and  Leverrier's  determinations  of  the  solar 
parallax,  leading  to  the  detection  of  another  small  error  in  the  latter. 

16.  Stonk,  1868— .¥.  X.  /.'.  .1.  -S'.,  xxviii.,  p.  25.5. 
Rediscnssion  of  the  observations  of  the  transit  of  Venus,  17G9.  Only 
observations  of  ingress  and  egress  at  the  same  station  are  used,  and  certain 
alterations  are  made  in  the  usual  interiirelalion  of  thi;  observations  by 
Chappo  in  California,  and  Captain  Cook  and  his  companions  at  Otaheite. 
The  result  of  these  alterations  is  that  the  parallax  is  increased  to  8".91. 


540  JPPENDIX. 

17.  Nfavcomb,  1808— .1/.  ,V.  It  a.  S.,  xxix.,  p.  6. 
Criticism  of  Mr.  Stone's  interpretation  of  Ch.appe's  observation  of  egress 
in  1709. 

18.  Stone,  1808— J/.  X.  li.  A.  S.,  xxix.,  p.  8. 
Kojily  to  tlic  preceding  pajier. 

19.  Fayk,  1869 — Comptca  Rcnilus,  Ixviii.,  p.  42. 
Examination   of  tlie   observations   and  interpretations   in   Mr.  Stone's 

paper,  conclnding  that  all  tliat  we  can  decide  from  these  observations  is 
that  the  solar  parallax  is  between  8".7  and  8".9. 

20.  Stone,  1869— j1/.  N.  E.  A.  S.,  xxix.,  p.  230. 

Reply  to  Faye,  criticism  of  Powalky's  i»aper,  and  fnrther  discnssions  hav- 
ing for  their  object  to  show  that  the  resnlts  of  his  pajier  agree  with  the 
scattered  observations  of  ingress  and  egress  in  Enrope  and  America. 

21.  Anonymous,  1809 — Vkrtcljahrsschrift  der  Ash:  GcseL,  iv.,  p.  190. 
General  review  of  recent  papers  on   the  solar  parallax,  dealing  more 
especially  with  the  work  of  Stone  and  Powalky. 

22.  PowALKY,  1870— Astr.  Xachr.,  Ixxvi.,  col.  161. 

From  a  second  discussion   of  the  transit  of  Veuns,  1709,  he  deduces 

8".7809. 

23.  PowALKY,  1871— Astr.  Xachr.,  Isxix.,  col.  25. 

From  the  mass  of  the  earth  as  given  by  the  niotion  of  the  node  of  Venus, 
8".77.  Put  the  ado])ted  mass  of  Venus  enters  into  the  result  in  such  a  way 
aa  to  make  it  decidedly  nncertain. 

24.  Levekuieh,  1872 — Comptes  Ucndus,  lxxv.,p.  105. 
Determination  of  the  solar  parallax  from  the  mass  of  the  earth  as  derived 
from  'he  motions  of  the  planets,  and  the  diminution  of  the  obliquity  of  the 
ecliptic.  Rcisult,  8".80.  (The  distingiushed  author  of  this  paper  does  not 
distinctly  state  in  what  way  ho  has  allowed  for  the  fact  that  it  is  the  com- 
bined mass  of  the  earth  and  moon  which  is  derived  from  the  perturl>ations 
of  the  planets,  while  it  is  the  mass  of  the  earth  alone  which  enters  into  the 
formula  for  the  solar  parallax.  His  presentation  of  the  formula'  seems  to 
need  a  slight  correction,  which  will  diminish  the  parallax  to  8".83.) 

25.  CoUNU,  1874-'7G — Annales  dc  VOJmrrafo'irc  de  Parh,  xiii. 

Redetermination  of  tiie  velocity  of  light,  leading  to  the  parallax  8'  .794, 
if  Struvo's  constant  of  aberration  (20".  445)  is  used. 


SYNOPSIS  OF  I'Al'EliS  ON  SOL. Ill   I'MiALLAX,  l"^54-":v.    541 

26.  Gallk,  1875 — Bnshtit,  M<tni!ichkc  <)'•  Jlervndt. 

"  Uober  ciiio  Hcsstiiimmiij^  dor  Soniitui  I'aiallaxo  ans  roncspoiKliieiKlfii 
Beohachtniigcn  dos  Plaiietcii  Flora,  iiii  Octoli^-r  uiid  Novciiibor  1H7;{."  Dis- 
cussion of  obsorTations  niado  at  uinc  iiorthcni  observatorit's,  and  the  Capt', 
Cordoba,  and  Melbourne,  in  the  southern  hoiuisphero.     Kesult,  8".t<73. 

27.  l*L'i.SKLX,  1875 — Comptcs  lienditfi,  Ixxx.,  p.  'XV,i. 
Couijiutatiou  of  four  contact  observations  of  the  transit  of  Venus  iu  1874, 

made  at  Peking  and  St.  Paul's  Island.     Kcsnlt,  8".879. 

28.  LiNOSAY  and  Gill,  1877— .1/.  ^V.  1{.  A.  S.,  xxxvii.,  p.  'MS. 
Reduction  of  observations  of  .Iniio  with  a  helioi'icter  at  Mauritius,  in 
1874.     Tlui  result  la  8" .765;  or  8".815  when  a  discordant  observation  is 
rejected. 


542  APPENDIX. 


IX. 

LIST  OF  ASTHONOMirAL  W(»UKS,  MOST  OF  WriTCII  HAVE  BEEN  COX- 
srLTED  AS  AL'TlIOUrriES  IN  THE  I'lJEl'AUATIOX  OF  THE  FUESENT 
WORK. 

The  following-  eoiuprisos:  1.  A  fow  of  the  loiuling  works  of  tlio  great 
astroiioiiUTs  of  the  past,  and  of  the  investigators  of  the  present,  arranged 
nearly  in  tliti  order  of  time.  In  the  ease  of  works  befon;  ISdO,  the  sup- 
posed date  of  composition,  or  the  years  within  which  the  autlnjr  nour- 
ished, are  given.  The  list  is  presented  for  the  benelit  of  thos(^  teachers 
and  students  who  wish  to  be  ac(]u:iinted  witli  these  anilioritics,  and  can- 
not refer  to  such  works  as  the  ISiblhurapItiv  Astrouomiqnc  of  Lalande,  or 
the  I'MJUdwa  ('<il(tln<inN  Libroniin. 

2.  Modern  telescopic  researches  upon  the  physical  aspects  of  the  planets 
which  have  beim  employed  in  the  prcpiiratiou  of  Part  III.  of  the  present 
work. 

;{.  Kecent  works  on  special  departments  of  astronomy,  which  may  be 
useful  to  those  who  wish  to  j)nrsne  special  subjects  with  greater  fuluess 
than  tliat  with  which  they  are  treated  iu  elementary  woiks. 

In  tile  lirst  two  classes  the  selection  is,  for  the  most  part,  linuted  to 
works  which  have  been  consulted  as  authorities  in  the  ]»reparation  of  this 
treatise.  In  the  case  of  lleveliiis,  however,  some  writings  are  added 
which  I  have  not  nsed,  nor  even  seen,  with  the  object  of  making  the  list 
of  his  larger  works  complete.  Writings  which  Jiave  ajjpeared  in  jteriod- 
icala  and  the  transactions  of  learned  societies  are  necessarily  omitted  from 
the  list,  owing  to  their  great  number. 

The  jtrices  given  for  some  of  the  oldei'  books  are  those  for  which  tiiey 
are  eonunonly  sold  by  anti(iuarian  dealers  iu  Liermany. 

D.c.  250.  AitisTAiK'Hus  :  Dc  Mit{jnilndhiihiin  it  Dhtantiis  Solh  li  Jauuv.    Pisa, 
\:^TZ.     iSil. 

A.l).  VJ).  PtoM'.mv,  ('rAri)K  :    MlirAAHi:  i;VNTASKi2i:    HI  MA.  uncommon- 
ly called  The  JlmiKjcHl. 

The  most  recent  edition  is  by  tlic  Alilx'  Iliihnii,  in  ({ro!;.  with  French 
triinsliition.  Two  vols.,  4tu.  Fiui^,  11513-' IU.  Connnouly  sellb  for 
§8  to  ■  10. 


LIST  OF  ASntONOMlL'AL    nOIlKS,  543 

880.      AuuTEGxns  :  Ik- Scknlia  StcUnrnm  rjlxr.     IJoim,  1G45. 

1543.      Corj;i!MCLs:  J)e  IlevolittioiiihHs  Orbiitm  Cahstiiiiii. 

The  tlrst  edition  of  the  great  work  of  C()i)eniicus  is  rare.  Tlic  secoiul 
(Basel,  \Mi)  sells  lor  ;?4.  Two  line  editions  have  been  published  in 
Gernuuiy  in  recent  times.     l*ri"c  .^T  to  sio. 

1597.      TYt:ii()    liUAUi:  :     Antronoiniw    Ittniaunitw    Michunica,       Noriborg, 
1G02.     .§3. 

Contains  description  of  Tycho's  instruments  and  rethods  of  observing-. 

Antro)i(>}iti<('  l)isl(iiir((lw  rnxjymiKtiiinatd. 

Jh    Mitndi   Jithcrcl  IhtTuiioribuH    I"  ■iuoiiicui'i.       l^rauk- 

foit,  KUt). 

Tliese  t.vo  volumes  ijonerally  i;"o  under  the  title  of  the  former.  A  later 
etlition  ( l(i4S)  was  i~Mied  uniler  the  misleading  tille  O^Jcra  Uiiinia.  Tin' 
selling'  price  is  st)  lor  the  two. 

1.590- Mvr.i'M'.H,  JoiLVXM'.s  :    Opera    Omnia.      Edidit   Dr.  Ch.  Frisch.      '^ 

1030.  S      vols.,  8vo.     Frankfort,  lH5H-'7 1. 

A  recent  and  complete  edition  of  Kepler's  vohiniinoiis  writings.  Price 
from  $^0  to  ><oU.     (ienerally  cheaper  at  second-hand. 

1590- ?  Gam i.Ko  (;alili;i:    Opcre.      13  vols.,  Hvo.      Milan,  1811.      Price 

IG3().  >       about  SlO. 

A  much  better  edition,  puldished  in  4to,  about  1845,  is  more  expensive. 
Galileo  wrote  almost  entirely  in  Italian. 

1603.      Havioi!,  JoiiAXNT.s:   r/aiioiiutriu. 

Buyer's  eelebratcil  star-eharts,  in  whicli  the  stars  were  first  named  with 
(JriH'k  letters.  Three  or  more  editions  were  published,  the  seeoiul  be- 
ing in  lt)4S,  the  third  in  KKil,     >;:i  .'jO. 

Rict'ioi.rs:  Almancslniii  Xontin.    2  vols,   n  one,  folio.     IJoun,  IG51. 

Antroiwmia  Hcformata.     Foli(».     Honn,  ir)()5. 

Two  ambitious  works,  rennirkable  rather  for  llnir  voluminousness  than 
for  their  value.  The  author  being  an  ecclesiar-tie,  had  to  profess  a  dis- 
beliif  in  the  Coi)crnican  systcn. 

1()30.       Ik'i-l-lALDls :  Aslronomia  I'liiloluird.     Folio.     I'aris,  1045. 

The  last  three  works  are  cited  as  iwobably  the  nu)st  voluminous  coin- 
pendinms  of  astronomy  of  the  seventeenth  century.  They  can  all  be 
piu'chasL  1  for  sJ:i  or  84  each. 

1011.       Faiuutii,.).:   IU   .Vacitlis  hi  Sole  ObxtrvaCn^. 

1055.       noKKi.i.i  :   Ik  f'iro  TdmojHi  Inrentore.     Hague,  1055.     $1. 

llKvr.i.n  s.  J. :  Svhno!in(2)hin,nir(  Lumv  Ik'Hcriptio.     Folio. 


1647- 
1690. 


The  earliest  great  work  on  the  geography  of  the  moon  and  llie  aspects 
of  the  planets.     Profusely  illustrated.    ?!4  to  ^o. 


544  APVENDIX. 

Hkvklu'.s,  J. :  Mcrciirhis  in  Sole  Visus.     Folio,  1002.     $1. 

Contiiins  also  llorrox's  observation  of  the  transit  of  Venus  in  1039. 

Vomvto(jrai)hUt .     Folio,  1GG8. 

The  first  j^rcat  modern  treatise  on  the  subject  of  comets. 

Machina  Ccclesiis,  Pars  Prior.     Folio,  107;}. 

Contains  (hiscriplions  of  liis  instruments,  and  a  disquisition  on  the  prac- 
tical astronomy  of  liis  time. 

Muchina  Calestis,  Pars  I'ontcrior.     Folio,  lG7i). 


A  v(;ry  rare  booi<,  almost  tlu;  entire  edition  having  been  destroyed  by 
lire.     A  copy  was  sold  for  )^M  in  lS7:i. 

Annus  Climuchricns.     Daiitzie,  1G85. 


Prodromiis  Astrononiice.     Diuitzic,  1090. 

Firnutmentnm  Sohicsciannm.     Dautzic,  1690. 

These  works  comprise  star-catalogues,  stur-maps,  etc.    5*3  50. 

1659.      IIiYGHEXs:  Systcma  Saturnium.     Hague,  1059. 
Iforolof/iuni  Oscillaiorium.     I'aria,  1073. 


The  latter  work  contains  tlie  theory  of  tlu'  pendulum  clock.  These  two 
niid  most  of  the  otiier  imiiortant  works  of  lluyi;liens  were  published 
in  Leiden  in  1751,  under  the  title  of  Optra  MivlKtnlca,  (icuinctrica,  As- 
triDioi/ika  et  Miseilkaica,  nominally  in  four  volumes,  but  tlie  jiaging  is 
eoutimious  throui^hout  the  series,  the  total  number  of  pages^  being 
770.     Leiden,  1751.     85. 

1687.      Newton,  Lsaac:  Philosophic  jSiatitralis  Principia  Matlumatica.    4to. 

London,  1087. 

A  number  of  editions  of  Newton's  Priudpia  have  appeared.  One  of  the 
n)()st  common  is  tluit  of  Le  Seur  and  Jacipiier,  3  vols,  in  4.  Geneva, 
1739.  It  is  accompanied  by  an  extended  coinnientary.  Sells  for  about 
^4.  A  very  fine  edition  was  issued  in  1871,  by  Sir  William  Thomson,  in 
Glasgow.  "  There  is  also  an  English  translation  by  Motte,  v.liich  has 
gone  tlirougli  several  editions  in  England  and  one  in  America. 

BuKWSTKH,  Siu  D.  :  ^f('moirs  of  the  Life,  Writinf/s,  and  Discoveries 
of  Sir  haae  Xvwton.     2  vols.,  8vo.     Edinburgh,  1855. 

1720.       Flamstki:i),  J.  :    Historia  Coclestis  Britanniva.     3  vols.,  folio.     Lou- 
don, 1725.     ^10. 
Contains  Flamsteed's  observations  and  star-catalogue. 

1728.      Blaxcuixi,  F.  :  llesperi  et  Phosphori  nora  Phwnomena  sive  Obscrra- 
tioncs  cirea  Plnnetam  Veneris.     Folio.     Rome,  1728. 

1740.  Cassini:  f:lemms<VAstronomie.     4to.     Paris,  1740.     $1. 

1741.  Wkidlioh,  Jo.  :    Historia  Astronomiw.      Small  4to.      Wittcmberg, 

1741.    i2. 

JiEUNOuiLLi,  John  :    Oj)era  Omnia.     4  vols.,  4to.     Lausanne,  1742. 


LIST  OF  AISTEONOMICAL    WORKS.  545 

Le   MoNMiiu:   La  Tht'orie  dcs  Vomiti)i.     1  vol.,  Hvo.     Paris,  1743. 

$1- 

1760.      Kant,    1m.ma.\i-i;l  :    Sdtrifkii     cwr    I'hi/.siticluii    (iaxjntphk.      r^vo. 
Leix)zi>;,  l^il5l). 

17dO.     riNciKt;:    Coinchxjntpliic ;   on   Truitc   Hialoriqitc  vl    ThcoriqHc  ckn  Co- 
melef<.     2  vols., 4t<).     I'aiis,  17H;{. 

The  must  complelc  historkul  uiid  i^eiicrul  tix'ulibu  on  (.(jiir'U  which  iuis 
appeared. 

nHO-  }  Ba.i.i.y  :  Ilistoirc  d<:  VAnlrononiie  Aurk'inw  (k'])>iin  son  Orkjiiic  jiisqii^) 
171)0.^       VLUthliii^iiiuiilckVPkvkd'Jkxandrk:    1  vol.,  4to.    I'aiis,  17f^l.   >iiu. 

Jfixlohc  df   rjslronoinic  Modo-tic  dciiitis  la    I'oiidalioii  dr 

I' Leak'  d\lk-j(iiidru,jiin(ju'it  I'Lj^ioquc  dc  AIDL'CXXX.     '6  vols.,  iCo 
Paris,  1779.     $(). 

Truitc  dc  V AslroHOiiik  Indknnv  cl  Orkiitak.     1  vol.,  4to. 

Paris,  1787. 

These  lilstories  by  Bailly  are  eoiisiclered  vi>ry  iiii-oiiiul.  the  author  hav- 
ing a  greatly  exaggerated  uiiiuiuu  ot  the  knuwledge  ul'  the  aiieieuts. 

1800.      Lai.anih;,  J.  Dk:    Jlihlioijniiihk'  Asfronomhiiic ;    airr  VIHnioirc  dc 
lAalronoinie  dcpuis  I7fil  juNim'd  If^M.     4to.     Pari.s,  18015.     ijji'S, 

1817.      Lai'LACF,,  W  S.  :    Trailc  dc  Mccankiuc  Ccksle.     4  vols.,  4to.     Paris. 
17t)y-18Ur).    |160. 

This  work  is  now  exi)ensive,  all  the  editions  being  exhausted.    A  new 
edition  is  soon  to  be  issued. 

ExpoHilion  da  Synicmc  da  Monde.     1  vol.,  4to.    S'i. 

The  latter  wi.rk  gives  a  very  clear  popular  expo.-ition  ot  the  laws  of  the 
celestial  motions. 

Dklamhki;  :  Ilistoirc  dc  VAatroHomk  Auciciiuc.     2  vols.,  4to.     Paris, 
1817.     84. 

Ilintoirede  VAstronomie  du  Aloycn  Afjc.     1  vol.,  4to.     Paris, 


1811).     $3. 

Ilhtoirc  dc  V Asiivnomk  Moilcnic.     2  vols.,  4to.      Paris, 

1821.     $5. 

Ilistoirc  dc  V Astronomic  <ut  dix-hiiiticnic  Sicck:  1  vol..  4to.  Paris, 
1827.    .s:i. 

Thes(!  histories  by  Delambre  consist  iirincipally  of  abstracts  of  the  writ- 
ings ol'all  eminent  astroiKiniers,  accuiM|>unied  by  a  runniui;  coiiunen- 
tary,  bnl  without  any  attempt  at  louieal  ai'iatiu'i'mcnt.  Kacli  work  is 
taki'ii  up  and  ])asscd  throuuli  in  reiiular  order,  l)ut  it  is  only  in  the  in- 
troductory essays  that  general  views  of  the  [jrogrcss  of  the  science  ari' 
found. 

36 


540  APPENDIX. 

Encke,  J.  F. :  /)(>  Enlfrniinifi  dcr  Sonne  ron  dcr  Erde  aus  dein  Ve- 
niinditrclKjiUu/r  von  17(11  hcnjilcitct.     12tM0.     Gotlia,  18"22. 

Dcr  l'cnns(ln/rliij(tn(j  ron  I7()l).     lyrno.     Ootlia,  1824. 

These  two  little  booUs  eoiiluiii  EiicUe'.s  rcseiirelies  on  the  sohir  ])iiralliix 
Ic'iuUiig  to  the  result  8".5770,  and  the  distance  of  the  sun  'J."j,oOU,U(i(i 
niilus. 

Idki.ku,  1)11.  Li'DWu;  :  Jlandbnch  dcr  Mdthcmatischcn  nnd  Tccltninchcn 

CItronohxjic.     xJ  vols.,  bvo.     IJerlin,  1825. 

An  exhaustive  luul  commendable  work  on  the  measures  of  time  adopted 
in  various  countries,  especially  in  ancient  times. 

WiiKWKLL,  Wm.  :  nititorij  of  the  Indnciivc  ScicnvcR.     London. 

Heusc'Hel.  Sin  John:  Besnlts  of  Astronomical  Olwrrations  made 
dnr\n(j  the  Years  18:54,  'o,  '0,  '7,  '8,  at  the  Cape  of  (lood  Hope.  I 
vol.,  4to.     London,  1847. 

StUi'VE,  F.  G.  W. :  Etndcs  d^ Astronomic  Stcllaire.  St.  Pctcr.sbnrg', 
1847. 

Grant,  Robert:  ITistorii  of  Phtjsiciil  Astronomi/,  from  the  Earliest 
Ayes  to  the  Middle  of  the  Xineteenth  Centnrij.     8vo.     London,  18r)2. 

BiOT,  J.  B. :  Etudes  snr  VAstrononiie  hidicnne  et  Chinoisc.  8vo. 
Turis,  1862. 

LovERiNO,  Joseph  :  On  the  Periodicitii  of  flic  Aurora.  Memoirs 
of  tlio  American  Academy  of  Arts  and  Sciences.  Boston,  1851) 
and  1805. 

Olhers,  W.,  and  Galle,  J.  G. :   Die  leiehtste  und  hcquemstc  Methodc 

die  Bahn  eines  Comcten  :n  herechncn.     8vo.     Lcipzij-',  18(54. 

This  work  contains  a  tabic  of  all  orbits  of  comets  eoniputed,  brought  \i\> 
to  the  end  of  ISOo. 

Zoi.i.NER,  Dr.  J.  C.  F. :  Eehcr  die  Xatur  der  Kometen.  8vo.  Lei])zijr, 
1872. 

DuiiRiNG,  Dr.  E.  :  Kritische  Gcschichtc  d(r  T^rincipien  dcr  Mechanik. 
8vo.     Berlin,  1873. 

ToDiU'NTER,  I.  :  Historji  of  the  Mnthemntical  Theories  of  Attraction 
and  the  Fir/ure  of  the  Earth,  from  the  Time  of  Xewton  to  that  of  La 
Place.     2vols.,8vo.     London,  1873. 

II._WORKS  ON  THE  PHYSICAL  ASPECTS  OF  THE   PLANETS. 

SciiROETER,  J.  ir. :  lieitrdflc  ch  den  Xeuesten  Astronomischen  Ent- 
dcckunficn.  IIerausijc(jehen  von  Bode.  3  vols.,  8v().  Berlin,  1788- 
1800.     .§5. 


LIST  OF  ASTRONOMICAL    WO.UKS.  547 

Sc'iinOKTKU,  .).  ir.  :    Sclcnotopofiraphisrhr   Frufirnvntc  znr  yenuutrn 
KniulnisH  (Icr  .)[on(ljUuh(:     4ti).     Liliciitlial,  IT'Jl.     $',\. 

AphrodUoijraphm'he  Frugmvute  zur  (jnnincrn  Kenntniss  des 

rUinvten  I'viiHs.     4to.     lU-husU'dt,  ITlKi.     $(). 

Sclirov^tcr's  style  was  iiitoU'rulfly  i)i'olix  aiul  ilitl'iiso,  so  tliut  a  clciir  idea 
of  the  I'L'siills  lie  really  altained  involves  no  small  labor. 

I5kk1{,  W.,  and  Madi.I'.h,  J.  II.:  I'lijishdif  Ilcubachlinifiiii  dcx  Afari 
bci  nrliicr  (tppimtion  hii  ScpUinhrr  Ir^'M).     I'Jiiio.      I>(  rliii,  l.~l>(). 

Ihr  Moiid   nuch  nviiirn   hoHmixrIivu   loid  iiidiridmlhii    Ver- 

hdltiiiswiijodtr  AUijcmcinc  vcfijlckhoidf  ScliiKxjrdpliii ,    41  o.    IJerliii, 

18:j7.    ijj;?. 

This  volume  is  accompanied  liy  a  lai-i^c  iiiai>  of  the  moon,  and  is  tlio 
most  complete  and  eeleljraletl  worU  on  selenugiaiiliy  wliieli  lias  yi't 
appeared. 

Bkkk,  W.,  and  Madlki:,  .).  H. :  Ik'driiijc  znr  phuH'iHchn  Kinnhim 
dcr  himmlitichcu  Kurpcr  iin  Soinioixiixttmi'.     4to.     Weimar,  1841. 

Z(3llneu  :  I'liotomctriHchc  Unlersiichuiuirii  init  hcwndercr  Iliicksivht 
(III/  die  phyxhvhc  BtsvltaffcnhiU  dcr  Ilimmclnkurpcr.  8vo.  Leip- 
zig, 18G5. 

ExdKl.MAXX  :  rchrr  die  JleUiijkeilnrrrhf'illiiisne  dcr  Jiipitemtrahanteii. 
8vo.     Li'iiizig,  1871. 

VoCKi,,  H.  ('.,  and  Lousr. :  Ucoharhtuwijen  amjcxUiU  aiif  der  Sleru- 
mirte  den  luimmerherrii  von  Biilow  za  Jiuthkamp.  3  i)ts.,4to.  Leip- 
zig, 1872-75. 

III.-RECENT  TKE.\TISES  ON  SPECIAL  SUBJECTS. 

Till:  Hex. 

Proctou,  R.  a.  :  The  Sun  :  Ilnlcr,  Fire,  Lifilit,  and  Life  of  the  Plan- 
etanj  Sjifitcni.    8vo.     London,  1871. 

LOCKYEK,  .T.  N. :  Conirihiitions  to  Sohir  PhiiHirH.    8vo,  liondon,  1874. 

Skcciii,  a.  :  T.e  Soldi     2  v(ds.,  8vo,  with  Atliis.     Paris,  187.'»-'77. 
The  latter  is  the  most  eomi'hte  and  beantifully  ilhistratcd  treatise  on 
the  sun  which  has  yet  appeared. 

TiiK  Moon. 
Nasmytii  and  CAHPKXTr.iJ :   The  .]fonn.     London,  1874. 
Contains  very  beautiful  illustrations  oflunar  scenery. 
PuocTou,  R.  A.  :    The  Moon  :    Her   Motions,  Aapcdf),  Scenery,  and 

I'hiixieal  Condition.     8vo.      London,  187:?. 
This  work  is  illustrated  with  several  of  Mr.  liutherfurd's  photographs. 


)48  APPENDIX. 

Nkison,  EPMrxi) :  The  Moon,  nnd  the.  Cond'dhm  and  Confifjumliona 

of  its  Suiface.     Illustrated.     Hvo.     Loudon,  l."<7(j. 
Priucipally  devoted  to  selenography. 

Transits  ok  Vkxus. 

FoKBES,  Georce  :  Transits  of  Venus.     Loudon,  1874. 

Proctor,  R.  A. :    Transits  of  J'enus.     A  Po})ular  Account  of  Past 
and  Coming  Transits.     8vo.     Loudon,  1875. 


THEORETICAL   AND   TRACTICAL   ASTRONOMY. 

LooMis,  Ei.iAS  :  An  Introduction  to  Practical  Astronoiiiji,  with  a  Col- 
lection of  Astronomical  Tables.     8vo.     New  York,  1855. 
Contains  nuieli  inforniation  for  the  amateur  astronomer. 

Sawitcii  :  Abriss  der  Practischen  Astronomic.     2  vols.,  8vo.     Ham- 
burg, 1850. 

Brunxow,  F.  :  Practical  and  Spherical  Astronomy.     8vo.     Loudou 
aud  New  York,  1805. 

CiiAUVEXET,  W. :  Manual  of  Spherical  and  Practical  Astronomij,     2 

vols.,  8vo.     riiiladel|diia,  18(13. 

The  most  complete  and  exluuistive  treatise  on  the  subject  which  has  yet 
appeared. 

Watsox,  J.  C. :  Theoretical  Astronomy.     8vo.     I'hiladelpliia,  1808. 


GLOS:SAIiy  OF  TECUyiVAL   TERMS.  541) 


X. 

GLOSSARY  OF  TEf'IINTCAT-  TEiniS  OF  FRE{,)T'ENT  OCCrURENTE  IX 

ASTRONOMICAL   \\(t|{KS. 

The  folio-wing  list  is  liolipvod  to  incliidc  nil  the  tfclmicnl  tci'iiis  used  in 
the  present  Avork,  as  well  as  a  nnniher  of  otiuis  wiiicli  iIk;  reaiU'i'  of  as- 
tronomical literature  Avill  iVe(iuently  meet  \vi(li.  Tiie  words  in  jtareuthe- 
scs  which  sometimes  follow  a  term  express  its  literal  sic;nilication. 

AbeiTation  {a  ivdudcr'niti-airan).  Generally  aitplied  to  a  real  or  apparent 
deviation  of  tii(^  course  of  a  ray  of  lij;ht.  Especially  (I)  an  aiiiiarctit 
displacement  of  a  star,  owinjj,'  to  the  progressive  motion  of  light  corn- 
Lined  with  that  of  the  earth  in  itsorhit,  ]).'211 ;  (2)  the;  defects  of  action  of 
a  lens  in  not  hriiigiiig  all  rays  to  the  same  focus.  'j"he  xithivlnil  nhnrdtlou 
of  a  lens  results  in  tiu;  rays  whicli  pass  tlirough  tiui  glass  near  its  edgi^ 
coming  to  a  shorter  focus  than  those  wiiich  ])ass  near  its  centre,  while 
the  chronKilic  (thcrrttliou  is  the  separation  of  tiie  light  of  dilVcrcul  colors. 

Achromatic  {irilhoni  cDlor).  Applied  to  an  ohject-glass  in  v/liich  rays  of 
(lilVcrtMit  colons  are  hroiight  to  the  same  focus.     See  ]>.  114. 

Aerolite.    A  metemic  stone  or  other  hody  falling  from  the  celestial  sjiaces. 

Albedo.  Dcgi-ee  of  whiteness,  or  iiro])ortion  of  incident  light  reflected  hy 
a  non-huninous  body.  When  the  allic<lo  of  a  hody  is  said  to  he  O.)!,  it 
means  that  it  rctlects -j^j^  of  the  incident  light. 

Alidade.  A  nntvahle  frame  carrying  the  niii  roscopes  or  verniers  of  a  grad- 
uated circle.     Not  generally  used  in  instruments  of  recent  constrnction. 

Altitude.  The  apitarent  angular  elev;ition  of  a  hody  above  tlui  horizon, 
n.snally  expr(>sscd  in  degrees  and  niiiiutes.  At  the  horizon  the  altitude 
is  zero,  at  the  zenith  it  is  Wf^. 

Annular  {y'Difi-nluijud).     Having  the  appearance  or  form  of  a  ring. 

Anomaly.  The  iingular  distance  of  a  ])lanct  fnmi  that  point  of  its  (U'hit 
in  which  it  is  nearest  to  the  sun,  or,  in  the  ancient  astronomy,  to  the 
earth.  Draw  two  straight  lines  from  the  sun,  oiu'  to  the  nearest  point 
of  the  orhit,  or  the  ])crihclion,  and  the  other  to  the  planet,  and  the  an- 
gle between  these  lines  will  be  the  iinomaly  of  the  ])lauet. 

Anomalistic.  I'ertaiuing  to  the  anomaly.  'i"li'-  anomalistic  year  is  the 
jjcriod  between  two  eon.secntivc  returns  of  the  earth  to  its  perihelion. 
It  is  about  4'  I't"  longer  ihan  the  sidereal  year. 


550  APPENDIX. 

Ansae  (liaiidlin).  Tlio  ;ipparont  ciuls  of  the  rings  of  Saturn,  ^vllic•h  look 
like  liandlcs  jnojcctinj;  IVoni  tlic  planet. 

Aperture  of  a  Telescope.  'J'lic  dianu'tor  of  the  glass  or  mirror  which 
admits  tin-  rays  of  ]i};l)t,  clear  of  all  obstacles. 

Aphelion.  Tiie  part  of  the  orliit  of  a  planet  in  which  it  is  farthest  from 
the  sun. 

Apogee.  'J'he  point  of  an  orbit  in  which  the  |»lanet  is  farthest  from  the 
earth.  In  the  ancient  astronomy  the  planets  were  said  to  be  in  apogee 
when  beyond  the  sun,  an»i  therefore  at  their  giu'atest  distance  from  the 
earth;  but  the  term  is  now  applied  only  to  the  most  distant  point  of 
tlu^  moon's  orbit. 

Apsis  (pi.  .//^.s/'f/cs).  The  two  points  of  an  orbit  which  are  nearest  to,  and 
farthest  from,  the  centre  of  motion,  c;illed,  respec^tivcly,  the  lower  and 
higlii'r  a^tsis.  'J"lu^  liin'  of  tip.sidcn  is  that  which  joins  tluise  two  points, 
and  so  forms  the  major  axis  of  an  elliptic  orbit.  The;  ter  is  now  near- 
ly superseded  by  ihe  more  special  terms  tiplicHoii,  pcrihcHo/i,  pcriijee,  etc. 
See  EUmcutfi. 

Arniillary  Sphere.  A  combination  of  circles  used  before  the  invention  of 
tiie  telescopi!  for  deternuuing  the  relative  directions  or  apparent  posi- 
tions of  the  heavenly  bodies  on  the  celestial  si)hcre.  It  is  now  cMitirely 
out  of  u,s(!.     See  ]».  1(1;'). 

Astrolabe.  A  sinij)le  forui  of  arniillary  sphere  used  by  the  ancient  as- 
tronomers. 

Azimuth.  Tiie  angular  distance  of  a  point  of  the  horizon  from  the  north 
or  south.  The  azimuth  of  .a  horizontal  line  is  its  deviaticm  from  the 
true  north  and  south  direction.  The  azimuth  of  the  east  and  west 
jtoints  is  DO  '. 

Binary  System.  A  double  star,  in  which  the  twct  coiuponenta  are  found 
to  revolve  round  each  other. 

Binocular  {(ivo-cjicd).  Applied  to  .a  telescope  or  microscope  in  which  both 
eyes  can  be  used  at  once,  as  an  opera-glass. 

Black  Drop.  A  distortion  of  Mercury  or  \'euus  at  the  time  of  internal 
contact  with  the  limb  of  th(>  sun.     See  p.  171*. 

Centesimal.  Keckoiiing  by  hundreds.  Applied  to  those  denominational 
systems  in  which  each  unit  is  one  hundred  «imes  that  next  below  it. 
The  centi  .limal  division  of  the  angle  is  one  in  which  the  (|uadrant  is 
divided  into  100  degrees  or  grades,  the  grade  into  100  minutes,  and  the 
minute  into  100  seconds. 

Chronograph  (time-mark).  An  instrument  for  measuring  time  by  marlc- 
iug  on  a  moving  paper  (see  p.  loo).  Time  is  then  represented  by 
sjjace  ]iassed  over. 

Circle,  Great.  A  circle  which  divides  the  sphere  iuto  two  equal  hemi- 
spheres, as  the  equator  and  the  ecliptic. 


(iLOSSAJlV  OF  TECllNWAL  TKUMS.  551 

Colures.     'I'lic  Inur  iiriiiiii)ul  iin'ritlinus  oftlio  ci-lcstial  siilicrc,  all  of  which 
'  pass  from  the,  pole,  and  (nm  ol'  wiiich  jtasst-s  tliroiij;li  each  t!(|iiiii(ix,  aiul 

Olio  throiij'li  ca.h  Milstico.    Tiu-y  mark  the  ciii'lcs  of  I)'',  (li',  12'',  and  IHi' 
of  rij;ht  ascciisiiiii.  icspc<'liv«'ly. 
Conjunction  [n  joiuiiKj).     'I'lic  iicaifst  aiii)ai(Mii  aiipioach  of  two  licavf'iily 
hodics  whifh  si'iiu  to  pass  each  other  in  tiicir  course.     'I'liey  are  coiii- 
luonly  considered  as  in  conjunction  when  they  have  the  same  h  ngilnde. 
The  term  is  applied  especially  in  the  case  of  a  planet  and  the  sun.     The 
iican'st  approach  i.s  called  superior  conjunction  when  th:'  planet  is  Xw- 
yond  the  sun,  infeiior  when  it  is  this  sid(!  of  i(.     Mercury  and  Veuus 
are,  of  course,  the  only  planets  which  can  he  in  inferioi- c(in,iuuc(ion. 
Cosnucal.      Kelatinj;  to  cieation  at  larj;e,  in  contradistiaciion  to  terres- 
trial, which  relates  to  the  earth.     ]5y  a  cosiuical  pheiiunu'non  is  meant 
one  which  litis  its  origin  outside  the  earth  aiul  its  atmosphere. 
Culmination.     The  [tassage  of  a  heavenly  body  over  the  meridian  of  a 
place.     This  jiassage  nuiy  In    considered  as  occurring  twice  in  a  <lay, 
once  above  the  pole,  and  again  below  it,  twelve  hours  later.     Tin;  ftn- 
incr  is  called  the  ujtpcr,  the  latter  the  /(»«•(/',  culmination.      The  u[iper 
culmination  of  the  sun  occurs  at  noon,  the  lowei'  at  midnight. 
Cusps  {))()iiits).     The  [lointed  ends  of  the  sei'nnng  horns  of  the  moon  or 

of  a  planet  when  it  presents  the  appeaiance  of  a  crescent. 
Cycle  {circle).     A  period  of  tinm  at  the  end  of  which  any  aspect  or  rela- 
tion of  the  lha\  t'uly  bodies  recurs,  as  tho  Metonic  cycle. 
Declination.     The  angular  distance  of  a  heaveidy  body  fiom  the  eijuutor. 
When  north  of  the  e(|uator,  it  is  said  to  be  in  mnth  declination;  other- 
wise, in  s(nith  declination. 
Deferent.     In  tho  ancient  astronomy  the  mean  orbit  (d'  a  planet  which 
was  supposed  to  carry  the  epicycle.     It  is  represented  by  the  dotted 
circles  in  Figs.  lU  and  11,  pp.  ort  and  '.VJ. 
Dichotomy  {u  cHttiiuj  in  Iwo).     Tin;  asjiect  of  u  planet  when  half  illumi- 
nated, as  the  moon  at  lirst  and  last  (puirter. 
Digit.     The  twelfth  part   (d"  the  diameter  of  th(>  sun   or  moon,  I'oriucrly 

used  to  express  the  magnitude  of  eclipses.     See  p.iiJ?. 
Dip  of  the  Horizon.     At  sea.tle  depression  of  the  ai>])arent  horizon  be- 
low the  true  level,  owing  to  the  height  of  the  observer's  eye  above  the 
water. 
Diiect  Motion.     A  mcdion  from  west  to  east  among  the  stars,  like  that 

of  the  planets  in  general. 
Eccentric.     In  the  ancient  astroimmy,  a  circle  of  which  the  centre  was 

displaced  from  tho  centre  of  motion.     Sec  p.  i2,  Fig.  13. 
Eccentricity.     See  JCIvments. 

Ecliptic.     The  apparent  path  of  the  sun  among  tiie  stars,  described  in 
Part  I.,  Chap.  I.,  ^  3.     See  p.  V.l 


552  APPENDIX. 

Egress  {a  f/oing  forth).  Tlio  end  of  the  apparent  transit  of  one  body  over 
another,  wiien  the  former  seems  to  leave  the  latter. 

Elements.  In  general,  the  dai  for  predieting  an  a.strononiical  plienome- 
non.  Especially,  the  (piantities  which  dc^tennino  the  motion  ofa  plan- 
etary body.  The  independent  elements  of  a  planet  are  six  in  nnmbcr, 
namely : 

1.  The  mr(tn  distmicp,  or  half  the  longer  axis,  .IP,  of  the  ellipse  in  which 
the  planet  moves  ronnd  th(^  snn,  the  latt(!r  being  in  the  focus  at  S. 

2,  The  eccentricUji,  tiie  ratio  of  the  distance  CS  between  the  centre 
and  focns  of  the  ellipse  to  the  mean  distance. 

These  two  elements  determine  the  size  and  form  of  the  elliptic  orbit 
of  the  planet. 


Fi(!.  112 — Diagr.im  illnstrrtting  elliptic  elements  ofa  planet. 

3.  The  longitnde  of  the  ascending  node,  which  gives  the  direction  of 
the  line  in  whicii  the  plane  of  the  orbit  intersects  that  of  the  ecliptic,  or 
the  angle  which  tiiis  line  makes  with  the  vernal  eqninox. 

4.  The  inclination  of  llie  plane  of  tiie  orbit  to  that  of  f  lie  ecliittir. 

Jj.  The  longitnde  of  fin;  ]»erih('lion,  /',  for  whicih  is  taken  the  longitnde 
of  the  node.  ;>/»«  the  angnlar  distance  from  the  node  to  the  perihelion, 
as  seen  from  tlie  snn. 

These  three  (pnintities  determine  the  position  of  the  orbit  in  space. 

fi.  The  mean  longitude  of  the  ])lanet  at  some  given  epoch,  or  the  lime 
at  which  it  passed  tiie  jteiihclion,  /'. 

To  these  six  the  time  of  revolution,  or  mean  angnlar  motion  in  a  day 
or  year,  is  usually  added  ;  but  as  this  can  always  be  determined  from 
the  mean  distance,  and  rice  vrrxa,  liy  Kejdei's  third  law,  the  two  are  not 
regarded  as  independent  elements. 

The  quantities  w(!  have  described  are  usually  represented  by  algebraic 
symbols,  as  ftdlows : 


o,  tlin  moan  diptance. 

e,  the  rrccntricity. 

«  or  S2,  the  loniritudo  of  the  node. 

t  or  <j>,  the  iiK'lir.ation. 


(T)  or  TT,  the  Iniifritnde  nf  the  poriliolion. 
t,  tho  mean  Inn^'itiKU'  at  !^()Im^  I'poch. 
n,  the  mean  motion. 
w,  the  dii<tiince  from  node  to  perihelion. 


GLOSSAIiY  OF  TECHNICAL  TERMS.  553 

Ellipticity.  Deviation  fnim  a  truly  cimilar  or  sijlierioul  form,  so  as  to 
bocoiiiti  an  ellipse  or  sjjheroid.  An  orltit  is  said  to  be  more  elliptic  the 
more  it  deviates  from  a  circle. 
Elongation.  Tlu!  apparent  an<;jular  distance  of  a  body  from  its  centre  of 
motion,  as  of  Mercury  or  Venus  iVom  the  sun,  or  of  a  satellite  from  its 
primary. 

Emersion  {a  combuj  out).  Tlio  reappearance  of  an  object  after  being 
ccliiised  or  otherwise  hiddtni  from  view. 

Ephemeris.  A  table  j^iviu";'  the  position  of  a  heavenly  body  from  day  to 
<lay,  in  order  that  observers  may  know  where  to  look  fur  it.  Ajjplied 
also  to  an  astronomical  almanac  j;iving'  a  collection  of  such  tables. 

Epicycle.  In  the  ancient  astrononiy,  a  small  circle  the  centre  of  which 
moves  round  on  the  circumference  of  a  larf^er  one,  es])ecially  tht^  eircle 
in  which  the  three  outer  i  'nets  seemed  to  perform  an  annual  revolu- 
tion in  consequence  of  the  re   ohition  of  the  eartii  arouiul  the  sun. 

Equation  of  the  Centre.  The  anj^ular  distance  by  which  a  planet  mov- 
ing;' in  an  ellipse  is  ahead  of  or  behind  the  mean  i»osition  which  it 
■would  occupy  if  it  moved  uniformly.  It  arises  trom  the  eccentricity  of 
the  ellipse,  vanishes  at  perihelion  and  aphelion,  and  attains  its  greatest 
value  nearly  half-way  between  those  points. 

Equation  of  Time.     See  j).  1(;4. 

Equator.  The  great  circhs  half-way  between  the  two  jiolcs  in  ilie  earth 
or  heavens.  Tiio  celestial  etpiator  is  the  line  ET  in  Fig. :{,  i>.  r.i.  See 
silso  pp.  02,  and  140, 147. 

Equatoreal.  A  telescope  mounted  so  as  to  fidlow  a  star  in  its  ajiparent 
diurnal  course,  as  descrilicd  on  p.  lli>. 

Equinox.  Either  of  the  two  i)oints  in  which  the  sun.  in  its  apparent  an- 
nual coni'st^  among  the  stars,  crosses  tlie  equator.  So  called  liecause  the 
days  and  niglits  are,  when  the  sun  is  at  those  points,  e([nal. 

Evection.  An  inequality  in  virtue  of  wliich  the  moon  oscillates  about 
1]     on  each  side  other  mean  position  in  a  j)eriod  of  :U  days  \[)  hours. 

Eye-piece,  of  a  telescope.  Tin-  small  glasses  nearest  to  tin*  eye,  which 
nnignify  the  innige.     See  pp.  110  and  IIH. 

Faculae  (xnudl  torchcn).  Groups  of  small  shining  sjiots  on  the  surface  of 
the  sun  which  are  brighter  than  other  parts  of  the  i)hotosphere.  Tiiey 
are  giiuerally  seen  in  tiie  neighborhood  of  the  dark  spots,  and  are  sup- 
posed to  be  elevated  portions  of  the  iiliotnsidiere. 

Pilar  {mode  of  Ihn'ad).     Applied  to  micrometers  made  of  spider  lines. 

Focus  ('(  li)ri)l(icc).  A  i)oint-  in  which  converging  rays  all  meet.  The 
focus  of  a  tclcscop(>  is  the  point  at  which  the  image  is  formed.    See  p.  KID. 

Geocentric,  deferred  to  tJie  centre  of  the  earth.  The  geocentric  posi- 
tion of  a  heavenly  body  is  its  position  as  seen  or  measured  from  the 
cartli's  centre. 


554  APPENDIX. 

Geodesy.  The  art  or  scicMice  of  nicasuriiig  the  earth  without  rol'erencc 
to  the  heiiveiily  bodies. 

Gnomon.  In  the  old  astroiioiny,  the  style  of  a  sundial  or  any  ol»jeet  the 
shadow  of  which  is  nieasur'Ml  in  order  to  learn  tlu;  position  of  tlie  sun. 

Golden  Number.  The?  number  of  (he  year  in  the  Metonic  cycle,  counted 
from  1  to  li>.     .Se(>  j).  48. 

Heliacal  {irhiUn;i  to  the  ««h).  Apitlicnl  in  the  ancient  astronomy  to  those 
risinys  or  settings  of  bright  stars  which  took  place  as  near  to  sunrise 
or  sunset  as  they  (;ould  be  observed. 

Heliocentric,  deferred  to  tin;  sun  as  a  centre.  Applied  to  the  positions 
of  the  heavenly  bodies  as  seen  from  the  siui's  centre. 

Heliometer.  An  instrument  in  which  the  objec^t-glass  is  sawed  into  two 
eiiual  parts,  each  of  the  parts  forming  an  inilej)eudent  image  of  a  lu-av- 
enly  body  in  the  focus.  When  the  two  parts  are  together  in  their  origi- 
nal iiusition,  these  images  coincide,  but  by  sliding  one  part  on  the  other 
they  niay  be  separated  as  iar  as  is  d(;sircd  for  i\\c  pur]ioses  of  measure- 
ment. It  is  much  used  in  (Jeruiany  for  measuring  distances  too  great 
for  the  a])plicatiou  of  a  lilar  nncrometer. 

Heliostat.  An  instrument  in  which  a  mirror  is  moved  by  clock-work  in 
such  a  way  as  to  retiect  the  rays  of  the  sun  in  a  fixed  direction,  notwith- 
standing the  diurnal  nu)tion. 

Heliotrope.  An  instrument  invented  by  (iauss  for  throwing  a  ray  of  sun- 
ligl\t  in  the  direction  of  a  distant  station.  It  is  nuicli  used  in  geodetic 
measurements. 

Hour  Angle.  The  distance  of  a  h(>avenly  body  from  the  meridian,  meas- 
ured by  th(!  angle  at  the  j>ole.  It  is  conmionly  expressed  in  time  by  the 
number  of  hours,  minutes,  etc.,  since  the  body  crossed  tlu!  meridian. 

Innnersion  ((( ^*/»/(.(//»(.7 /*0-  i'l'*'  disappearance  of  a  body  in  the  shadow 
()f  another,  or  behind  it. 

Inclination,  of  an  orbit.     See  /'Jlcinciits. 

Ingress  (r(  (jo'nii/  in).  The  connuencemeut  of  the  transit  of  one  body  over 
the  face  of  another. 

Latitude.  The  angular  distance  of  a  heavenly  body  from  the  Co'Iiptic,  as 
declination  is  distance  from  the  c([nator. 

Libration  (<»  Nioir  siriiiiiiiH/.  <ts  of  o  hulniicc).  The  seeming  slight  oscillations 
of  the  moon  around  l.er  axis,  by  Avhich  we  sometimes  see  a  little  on  one 
side  of  her,  and  somctiuu's  on  Die  other. 

Longitude.  If  a  perpemlicnlar  l)e  dropped  from  a  body  to  the  ecliptic,  its 
celestial  longitude  is  tln^  distauct!  of  the  foot  of  the  iierpendicular  from 
the  verinil  e<|uinox  counted  towards  th»>  east. 

Lunation.  'J'he  ])cri(id  from  om;  change  <il'th(  moon  to  the  next.  Its 
duration  is  'i\)\  days, or,  more  exactly,  iiU.olJt*.')."^/'.!  days. 

Masa,  of  a  body.     The  quantity  of  matter  contained  in  it,  as  measured 


GLOSSARY  OF  TECHNICAL  TERMS.  00 5 

liy  its  \v(!ij'hL  at  a  ffivcii  placo.     Mass  din'cr.s  (Voin  wcij;lit  in  tliat  thti 
latter  is  ditVctit'iit  in  (litlfroiit  places  even  for  (lie  same  body,  (leju;n<lin;^ 
on  the  intensity  of  gravity,  whereas  the  muas  of  a  body  is  necessarily  the 
same  (everywhere. 
Mean  Distance.     See  Element n. 

Meridian.  The  terrestrial  meridian  of  a  place  is  the  north  aiKi  sontli 
vertical  plane  passing  tlirongh  that  ]>laee,  or,  the  great  circle  in  which 
this  plane  intersects  the  celestial  sphere.  It  i)asses  tlnongh  the  pole, 
the  zenitli,  and  the  north  and  south  points  of  the  liorizon.  Celestial 
meridians  are  great  circles  })assing  from  one  ]  ole  of  the  heavens  to 
the  other  in  all  directions,  as  shown  in  Fig.  4.").  \\.  117.  Every  celes- 
tial meridian  coincides  wl.,h  the  terrestrial  meridian  of  some  point  on 
the  earth. 
Metonic  Cycle.     See  p.  48. 

Micrometer  {umall  mmsurer).     Any  instrument  for  the  accurate  measure- 
ment of  very  small  distances  or  angles. 
Nadir.     Ti-    point  of  the  c(!lestial  .sphere  directly  l)eneatli  our  feet,  or  the 

direction  exactly  downwards. 
Node.     The  point  in  which  an  orbit  intersects  the  eelii)tic,  or  other  plane 

of  reference,  fiee  Elements,  and  p.  vJI?. 
Nutation.  A  very  small  oscillation  of  the  direction  of  the  (>arth's  axis. 
It  arises  from  the  fact  that  tlx;  forces  which  ])rodnce  tiie  ]>reccssi()n  of 
the  e(piinoxes  do  not  act  uniforndy.  and  may  tlicrcfoic  be  considered  as 
the  inequality  of  precession  arising  from  tiic  inecinaUty  of  the  force 
Avliich  produces  it. 
Oblate.      Applied  to  a  round  body  which  differs  from  a  sphere  in  being 

llatti'ned  at  tlie  i>oles.  as  in  the  case  of  the  eailli. 
Obliquity  of  the  Ecliptic.     The  inclination  of  tiic  plane  of  the  e([uator 
to  that  of  the  ecliptic,  which  is  e(|ual  to  hall'  t  lie  di  tic  re  nee  l»et  ween  the 
greatest  meridian  altilnde  of  the  sun,  which  occurs  about  .lune -ilst.  and 
the  least,  which  occurs  about  l)ecend>cr  'ilst.     At  the  begii'.ning  of  lf^.')0 
its  value  was  about  2',i^  27A',  and  it  is  diminishing  at  the  rate  of  about 
47"  i)er  century. 
Occaltation  (((  hUthuj).     The  disappearance  of  a  distant  body  through  tlm 
interposition  of  a  nejirer  one  of  greater  angular  nnignitude.     Applied 
especially  to  tlu'  case  of  th"  moon  passing  over  a  star  or  i)laiu't,au(l  to 
that  of  Jupiter  hiding  one  of  his  satellites. 
Opposition.     Tiie  relation   of  two   bodies   in   opposite  directions.     The 
itlanets  ar(>  saitl  to  be  in  opposition  when  their  longitude  dill'ers  l.-^O 
from  that  of  the  sun,  so  that  tiny  ri.<e  at  sunset,  and  set  at  sniuise. 
Orbit.     The  ]tath  described  by  a  planet  around  the  siiii,  or  by  a  satellite 

around  its  primary  planet. 
Parallax.     The  dill'crenee  of  direction  of  a  heavenly  body  as  seen  from 


556  APPExnix. 

two  iinints,  as  tlic  coiitro  of  the  earth  and  some  point  on  its  surface. 

SeePart  II.,  Chap.III.,^^  1. 
Parallels.     Iniaji'lnary  circles  on  the  earth  or  in  the  heavens  parallel  to 

tiic  equator,  and  liaving  the  poh;  as  their  centre.    Tiie  parallel  of  40°  N. 

is  one  which  is  everywliere  4U    from  the  ecpiator  and  5U    from  the  north 

pole.     See  Fig.  4;),  p.  147. 
Feiuiiubia.      A  ])artial  shadowing.      Applied  generally  iu   cases  where 

light  i.s  jmrtially,  hut  not  entirely,  cut  oh'. 
Peii-  {iiair).     A  gtnieral  i)rclix  to  (UMinto  tli(,'  i)oint  at  which  a  hody  rev(dv- 

iiig  in  orbit  comes  nearest  its  centre  of  motion  ;  iiHjpcrihdioii,  the  point 

nearest  the  sun;  prvUjcc,  that  nearest  tlie  earth;  peri- Saluruiiiiii,  thsit 

nearest  the  planet  Saturn,  etc. 
Perturbation.     A  disturbance  in  the  regular  ellii)tic  or  other  motion  of  a 

heavenly  body,  produced  by  some  force  addiliojial  to  that  which  causes 

its  regular  motion.     The  perturbations  of  the  planets  are  caused  by 

their  attraction  on  each  other. 
Photometer  {li(ihl-))ic(iniircr).     An  instrument  for  estimating  the  intensity 

of  light.     The  number  of  kinds  of  photonu'ters  is  \ery  gi'cat. 
Precession  of  the  Equinoxes.      A  motion   of  the  ])ole  of  the  equator 

around  that  of  the  ecliptic  in  about  !2(),tKlO  years.     >See  pp.  19,02,88. 
Prime  Vertical.     The  vertical  circle  passing  dm;  east  and  west  thnnigh 

the  zenith,  and  therefore  intersecting  the  hoi'izoii  in  its  east  and  west 

l)()ints. 
Quadrature.     The  positions  of  the  moon  when  she  is  1)0°  from  the  sun, 

and  then-fore  in  her  first  or  last  «iuarter. 
Radiant  Point.     That  ])oint  of  the  heavens  from  which  the  meteors  all 

seem  to  diverge  during  a  meteoric  shower.     See  p.  'MM. 
Refraction  {ti  hrntliiiifi).     The  bending  of  a  ray  of  light  by  passing  through 

ji  medium.     Asln»ii>mic(il  rcfi'<t<ti»»  means  the  rcfiaction  of  the  light  of  a 

heavenly  body  cause<l  by  the  ;itni(ispheri',  ;is  descriited  on  p.  :500. 
Retrograde  {hnrkwnfd).     Applied  to  the  motion  of  a  planet  from  cast  to 

west  .'nnong  the  stars. 
Saros.     A  period  or  cycle  of  18  years  11  days,  in  which  eclipses  recur. 

See  ]). :?(). 
Scintillation  {n  tinnlliufi).     The  twinkling  of  tlie  stars. 
Secular  {irhiliiKj  lt>  the  oijck).     Apjilied  to  lliose  changes  in  the  ]danetary 

orbits  which  require  immense  jieriods  for  their  completion.     See  \\.  Ih"). 
Selenography.     A  description  of  the  surface  of  the  moon,  as  geography  is 

ji  description  of  the  eaitirs  surface.     We  might  call  it  lunar  geography 

but  for  the  etymological  absurdity. 
Sexigesimal.     Counting  by  sixties,     Apjdied  to  those  denominate  sys- 
tems in  Avhich  one  unit  is  sixty  times  the  next  iiifcri»»r  one,  as  the  nsual 

Bultdivisiou  of  time  and  arc. 


GLOSSARY  OF  TIXUXICAL   TERMS.  557 

Sextant.     Tho  sixth  part  of  ii  circunifereiicc.     Also  an  iiistniiiu'iit  iiuaii 

used  iu  practical  astronomy  and  navigation,  for  the  roady  nicasurenicut  of 
.  the  anguhir  distance  of  two  points,  or  of  tho  ijltitndo  of  a  licavciily  body. 
Sidereal.     Relating  to  \\w  stars.     SUhrvul  time  is  time  measured  l»y  the 

diurnal  revolution  of  the  stars.     Each  unit  of  sidereal  time  is  about 

jj^th  part  uhorter  than  the  usual  one.     See  p.  150. 
Signs  of  the  Zodiac.     Tiu'-  twelve  eiual  parts  into  which  the  ecliptic  or 

zodiac  was  divid.'d  by  tiie,  ancient  astronomers.      These  signs,  begiu- 

uiug  at  tho  vernal  e(iuiuox,  are : 


Ariex,  the  Uain. 
TanniH,  llic  Hull. 
Gemini,  the  Twins. 
Cioiccr,  the  Crab. 
Leo,  the  Lion. 
Virijd,  the  Vir^'iu. 


Libra,  the  Balance. 
Scitrpiiix,  the  Seni'pion. 
SnijittariiM,  tho  Archer. 
Cai>riconiU!i,  the  (ioat. 
A'luariiiK,  the  Water-bearer, 
i'/.sct'.v,  the  Fishes. 


Solstices  (siaiidinfi-poinis  of  the  sun).  Those  points  of  the  e(li|ifie  which 
arc  most  distant  from  the  equator,  and  througli  wiiicb  tiie  sun  passes 
about  June  '21st  and  Dcjcember  *2ist.  So  called  because  Ibe  sun,  baving 
then  attained  its  greatest  declination,  stops  its  motion  in  decliiiatiou, 
aud  begins  to  return  towards  tho  eijuator.  The  two  solstices  are  desig- 
nated as  those  of  snminer  and  wint(!r  resi)ectively,  the  lirst  being  iu  (i 
hours  and  the  second  iu  Irt  hours  of  right  ascension. 

Sotluc  Period.  That  in  which  IIks  Egyptian  year  of ;{(!.')  days  correspond- 
ed iu  succession  to  all  the  seasons.  Tiie  e(|uiiioctial  year  being  supposed 
to  he  3G5.^  days,  this  period  would  be  1401  years,  but  it  is  really  lougci". 
See  p.  47. 

Speculum  ((t  mirror).    The  concave  uurror  of  a  rellectiug  telescope. 

Stationary.  Apjilied  to  those  aspects  of  the  planets  occurriug  between 
tho  periods  of  direct  and  retrograde  motion  when  they  apitcar  lor  a  short 
time  not  to  move  relatively  to  the  stars. 

Synodic.  A])plied  to  movemcMits  or  iieriods  relative  to  the  sun.  The 
synodic  niovenuMit  of  a  planet  is  the  auKuint  by  wliicli  its  motion  ex- 
ceeds or  falls  short  of  that  of  ihe  earth  round  tlu^  sum,  while  its  synodic 
period  is  the  time  which  elapses  betweeu  two  consecutive  returns  to 
inferior  or  sujierior  coujunetioii,  or  to  opposition. 

Syzygy.  The  points  of  the  moon's  orbit  in  whicii  it  is  eitiier  new  moon  or 
full  moon.  The  line  of  the  syzygies  is  that  which  passes  through  these 
points,  crossing  tlui  orbit  of  the  moon. 

Terminator.     The  bounding  line  betweeu  light  and  darkness  on  the  moon 

or  a  i)Iaiu't. 
Transit  {«  payn'oKj  dcross).    Th<^  passage  of  an  object  across  some  fixed  line, 

as  the  meridian,  for  example,  or  betweeu  the  eye  of  an  observer  aud  an 

apparently  larger  object  beyond,  so  tliat  the  nearer  objeet  ajtpears  on 

the  face  of  the  more  distant  one.    Applied  especially  to  passages  of  Mer- 


558  APPENDIX. 

ciiry  and  Vcinis  over  Uic  disk  of  tho  sun,  and  of  the  satellites  of  Jupiter 
over  tiio  disk  of  tlio  planet. 

Trepidation.  A  .slow  oscillation  of  the  ecliptic,  having  a  period  of  7000 
jear.s,  imagined  l)y  the  Acahian  astronomers  to  account  for  the  discord- 
ance in  the  deterniinations  of  Iho  itrccessioii  of  tlie  (^(luini^xes.  In  con- 
sequence of  this  motion  .lie  eqnino.^  was  supposed  to  o-scillate  backward 
and  forward  through  a  8])aco  of  ahout  twenty  degrees.  Tlie  trepidation 
continued  to  li<!;Mre  in  astronomical  tables  until  tlu^  end  of  the  sixteenth 
century,  but  it  is  now  known  to  have  no  foundation  in  fact. 

Umbra  {a  shadow).  That  darkest  part  of  the  shadow  of  an  object  where 
no  part  of  the  luminous  object  can  bo  seen.  Also,  the  interior  and  dark- 
est part  of  a  sun-spot. 

Vertical,  Angle  of.  The  small  angle  by  which  the  real  direction  of  the 
earth's  cenlie  from  any  ])oint  im  its  surface  dilfers  from  that  which  is 
directly  downward,  as  indicated  by  tho  plumb-line.  It  arises  from  the 
elipticity  of  the  earth,  vanishes  at  tin;  equator  and  poles,  and  attains  its 
greatest  value  of  about  1'2'  at  the  latitiule  ot'AW. 

Vortex  («  wliirlpoul);  pi.  J'orlircH.  Tlie  theory  of  vortices  is  that  which 
assumed  the  heavenly  bodies  to  be  carried  round  in  a  whirling  liuid. 
See  p.  72. 

Zenith.  The  point  of  the  celestial  sphere  which  is  directly  overhead,  and 
from  wjiich  a  plnnib-line  falls.  The  ticocriilric  :;cnilh  is  the  jtoint  in  which 
a  straight  line  rising  from  the  centre  of  the  earth  int(!rsects  the  celestial 
sphere.  It  is  a  little  nearer  tlie  celestial  equator  than  the  apparent  or 
astronomical  zenith,  owing  to  the  ellipticity  of  the  earth.  kSee  Vertical, 
Aiitjlc  of. 

Zodiac.  A  belt  encircling  the  heavens  on  each  side  of  the  eeli])tic.  within 
which  the  larger  planets  always  remain.  Its  breadth  is  generally  coii- 
sidi'red  to  be  about  sixteen  degrees — eight  degrees  on  cacli  side  tho 
ecliptic.  In  the  older  astronomy  it  was  divided  up  into  twelve  parts, 
called  niyvh  of  lite  zudiac. 


I  N  D  E  X  . 


A  bhe,  {li=triliut  ion  of  the  nebnlm 4.')'.* 

puntlliix  iif  SiriuH BiJG 

A  herratUm  of  lis'it  descfibeil 211 

AeccU'vatUni  of  moon's  motion 9(i 

Adams  ilctoi'niiMos  moon's  iircclci'Mtion.    !)0 

invesliLjates  motions  of  Uramis 360 

ylr>-o/i7w,  (lescripiion  of 3SC,  3SS 

Airii.  his  wiitoi'  teli'scoi)o 214 

density  of  tlie  enrtli 46 

Algol  a  vaiiable  star 426 

Apraritiiin,  circle  of  porpctnal 11 

Argdander  cataloi^'ucs  tlio  stars 414 

Argnu,  ^,  a  variable  star 42S 

Arixtan-hxta  allcmpts  to  measure  the  dis- 
tance of  the  sun 22 

^sA'j!,  motion  of  Encko's  comet 3S2 

Anteriiidn  (see  also  I'lanctn,  small) 323, 530 

Astrolabe  described 105,  54tl 

Astroii(/)iicr  Uo} ill,  duties  of 160 

Attraction  of  a  mountain 85 

of  small  masses 81 

Aurora,  description  of 301 

heiiilit,  nature,  etc 304 

periodicity  of 249 

spectnnu  of. 305 

Auivcrii,  motii)n  of  Sirius  and  Procyon. .  439 


/?a?7(/ determines  density  of  earlii S4 

Bailji'H  beads  exi)lained 314 

Barkvr,  spectrum  of  Aurora 305 

liaiicr  system  of  iiamiiii;  stars..  415 

Ileniinitti  (J.)  sustains  theory  of  vortices,    SO 

Z>V.s,sW,  parallax  of  61  Cy^ni 20C 

Dlack  drop  in  transits  of  Venus 179 

its  cause 1^1 

Bhmchiiu,  liis  irreat  telescope 112 

rotation  of  Venus 201 

y?od('',s1aw  of  ])lanetary  distances 233 

Bond  discovers  salelliie  of  Saturn 350 

intensity  of  moonliLrlit 317 

invest  igatcs  rings  of  Saturn 350 


/Jont-s,  list  of,  for  reference 542 

Ihwllcg  attacks  stellar  parallax 204 

detects  aberration  of  lii;lit 211 

nrnlic  (Tydio'),  his  obs.  nnd  system 6(( 

llriiinwir,  researclies  in  stellar  parallax..  20S 

Calendar,  history,  etc 44 

-Julian  and  (ireL,'orian 4'.> 

C'.inmirainian  telesco|>e 124 

Casm'ni  discovers  satellites  of  Saturn. . . .  352 

theory  of  Saturn's  rinL,'s 350 

Carindifih,  density  of  ilie  earth 82 

Caiilrii  determines  moon's  accelciation,.     98 

Challia  searches  for  Xeplune 36(» 

ChromoHjiherc  of  the  sun 25(> 

its  violent  movements,  etc 262 

Chronograph  described 155 

Chrh'fi  of  the  celestiiil  sphere 14(i 

Clurh  {.\1  van),  his  telesropes l;{7 

discovers  companion  of  Sirius 138 

Cln>iti<rn  of  stars 441 

Comet,  great,  of  KWO 374 

of  16S2  (Ualley'.'-) 375 

of  1S43 371) 

its  near  approach  to  sun...  259 

of  1S5S  (l)onati's) 310 

views  of 36'^,  380 

of  Hiela 37S,  396 

of  Kncke 381 

Couu'tn,  aspects  of,  etc 365 

develo|)meut 36»i 

relations  to  meteors 391 

motions 309 

number 373 

(Hbits  of,  their  form 309 

physical  ccuislilntion  of 398 

remarkable,  description  of 374 

tails  of,  repelled  by  the  sun 400 

ConittiUatioiix,  antiipiity  of  names 414 

description  of 417 

Copernicus  founds  modern  astronomy...    51 


560 


INDEX. 


Coj)ermcns  pnbliphps  his  system 5ii 

his  sj'stt  .1  •.x|)l:iiiH'il ^4 

repivsents  ecreiitriciiy  of  orl)it3 00 

his  clistnncos  of  the  planets G(l 

csliniatu  of  liis  woik (il 

worlv  foiuleiuiiL'd  by  Iiuiiiisilion 7'2 

Cotiiit  nicasures  velocity  of  light 21S,  '220 

Cunina  of  the  sun  described -IWi, 

its  probable  nature '25S 

its  spectium 257 

Cosmotjnnii,  the  system  of 401 

Ci]ck\  the  Metouic 4S 

Dean  determ.  transatlantic  longilnde 159 

Iktaunan,  secular  acceleration  of  moon..    97 

Dennitij  of  the  earth 84 

Di'scarten'  ttieory  of  vortices 72 

Dumiti's  comet,  description  of. ;!79 

views  of 36S,  JHO 

Dnqxr,  his  j;reat  telescope i;!5 

pliotoyraph  of  tlie  moon .Slii 

theory  of  the  solar  spectrum 002 

Earth,  density  of S4 

tigure  of,  view  of  Ptolemy ;!2 

on  Newton's  tlieory so 

the  Fropch  investigations.    S" 

theory  of  its  lluidiiy 299 

difliculties  of  tliis  theory 300 

temi)erature  of  interior 29S,  511 

secular  cooling  of. 511 

Easter,  how  determined 48 

Eaxt]iiaH,  view  of  total  eclipse  in  1SC9. . .  253 

Eccentric  in  ancient  astronomy 41 

EclipucK,  geometrical  ex))lanalion 24 

classitlcatiou 25 

duration  of 2S 

seasons  and  periodic  recurrence  ....    29 

total,  ])henomena  of 2,52 

of  1S09,  general  view  of 2,'')3 

observations  of. 257 

Ecliptic,  description  of 15 

obliquity  explained (Jl 

Elevtcutx  of  the  i)lanetary  orbits ,528.  ,5,52 

Enckc  determines  solar  i)anillax isl 

investigates  resisting  medium 379 

Epicijclen,  ancient  system  of 37 

explained  by  Copernicus .54 

Eiptator,  celestial 12, 147 

Evection  discovered  by  Ptolemy 43 

Eye-piece  of  telescojie 118 

Faculce  of  the  sun 5.53 

Fai/e,  constitution  of  the  sun 273 

his  comet,  motions  of 3S3 

Fizeau  measures  velocity  of  light 217 

Foucault  measures  velocity  of  liyht 218 


VM>r. 

Gatnxii,  or  Milky  Way,  its  aspect 410 

(Jaliico  reinvents  the  telescope 100 

discovers  i)hases  of  Venus 290 

satellites  of  Jupiter 336 

resolves  the  .'\lilky  Way 408 

Gallc,  parallax  of  asi<'roi(ls 2(K) 

optical  discoverer  of  Neptune 301 

(!c)itil,\i\!i  unfortunate  voyage ISO 

Gillinn.  expedition  to  Chili 174 

Glacial  epoch,  its  possible  cause 241 

Glanciiapp,  velocity  of  light 214 

Gnimwii,  its  use  by  the  ancients 104 

Githlcn  number 48 

Giiuld  determ.  transatlanfic  longitude...  159 

Gracitutiiiii  not  newly  discovered 48 

how  generalized  by  Newton 70 

universal  law  of 81 

exerted  by  small  masses 81 

explains  nuition  of  the  planets...  98, 100 

Grubb  constructs  Melbourne  telescope..  132 

Hull  observes  spot  on  Saturn 341 

IJallcii  discovers  secular  accel.  of  moon.  90 

total  eclipse  in  1715 2.52 

periodicity  of  his  comet 375 

proposes  obs.  of  transit  of  Venus  ...  170 

Ilaiisiii,  moon's  secular  acceleration  —  97 

solar  parallax 182 

llarlcitcsx,  si)ectriim  of  the  corona 257 

observes  meteoric  shower 389 

Ilcrschcl,  his  telescopes 126 

ditcovery  of  Uranus 3.54 

of  two  satellites  of  Uranus 3,55 

his  star  gauges 460 

struct  lire  of  the  universe 46S 

nebular  hypothesis 495 

Song  of  the  Telescope 127 

//%((/■(/  determ.  transatlantic  longitude.  1.59 

Ilipparcluts  observes  motions  of  jjlanets  40 

catalou'ues  the  stars 413 

IloMcn  investiiraies  satelliles  of  Uranus.  .3,57 

HtKikc,  jjroblem  of  stellar  parallax 203 

llorrox  tlrst  observes  transit  of  Venus.. .  175 

IIu<)(iii>s,  appearance  of  sun's  surface. . . .  239 

motion  of  stars  in  line  of  sight 450 

spectrum  of  nebiihe 447 

of  new  star 435 

Ilwjtilieiis  i)rcp.  the  way  for  gravitation.  73 

discovers  rings  of  Saturn 342 

Inquisition  condemns  work  of  Coperni- 
cus   72 

Intra-Mcrcnrial  ])lanets,  supposed...  100,281 

l)retended  t)bservalions  of 2S7 

Jan/ten  sui)posed  inventor  of  telescope. .  107 

Jansdcn  aualyjoes  solar  protuberances  . . .  254 


INDEX. 


561 


Ju2)iter,  the  planet 331 

appearauce  of  surface 332 

light  and  activity  of 33t 

rotation  of,  on  axis 3;i5 

eatcllitea  of 330 

Kant,  strurture  of  the  universe 462 

founds  uel)uln  -  hypotlic.«is 493 

KepU'.-  ii'vostiiraies  motions  of  planets..  GS 
first  two  laws  of  planetary  motion. .    09 

third  law 70 

structure  of  the  universe 401 

Lambert,  structure  of  the  universe 405 

Laiiijli'ii,  appearance  of  the  sun 238 

heat  of  the  sun 239 

on  the  sun's  constitution 2S0 

Laplace,  caui<e  of  moon's  acceleration. . .    % 

nebular  hypotliesis 495 

LaancU,  his  great  telescopes 131 

discovery  of  satellites 350,  304 

Latitnti<',bn\\  determined  astronomically  14S 
LeverrUr  investigates  motion  of  Mercury  100 

discovery  of  Neiitune 359 

Libration  of  the  moon 30T 

Light,  motion  of 210 

time  of  c(miing  from  sun 213 

velocity  of,  measuied 215 

Lippcrhvij  an  inventor  of  the  telescope. .  107 
Lockyer  analyzes  sun's  protuberances. . .  2.55 
Loivjitxiik,  terrestrial,  how  found 1.50 

the  transatlantic 159 

Loomis,  periodicity  of  the  aurora,  etc 249 

LoveriiKj,  periodicity  of  tlie  aurora 249 

Lyman  investigates  atmosphere  of  Venus  294 

Mars,  the  planet 320 

aspect  of. 321 

maps  of. 322,  323 

rotation  of. 322 

Maakclyue,  attraction  of  mountain 85 

Maxivelt,  tlieory  of  Saturn's  rings 3.50 

Mercury,  the  planet 2;s3 

ancient  theory  of 40 

aspect  and  rotation 2S4 

motion  of  perihelion 100,280 

transits  of 2^5 

Merulian  circle  described 152 

Meteoric  showers 385 

radiant  point  of. .390 

produced  by  comet 390 

Meteors  and  shooting-stars 3s4 

how  caused 387 

combustion  of,  by  motion 388 

orbits  of 394 

relations  to  comets .  391 

Metmiic  cycle 4S 


MUky  Way  described 410 

Miiller,  motion  of  Faye's  comet 383 

Month,  origin  of 45-47 

Moon,  revolution  and  phases 21 

acceleraf'ni  of  its  motion 90 

unexplained  changes  of  motion 98 

path  among  the  stars 23 

nodes,  motion  of. 23 

eclipses  of,  how  caused 24 

gravitation  of,  found  by  Newton.. . .     70 

investigations  of  the  ancients 42 

atmosphere 314 

surface  described 311 

distance  and  magnitude 300 

figure,  rotation,  and  libration 307 

changes  of  surface 310 

light  and  heat  of. 317 

effect  on  ihe  earth 319 

Miisic  of  the  spheres 4 

Xasmyth,  appearance  of  the  suu 23S 

Xebula',  appearance  of 444 

views  of 448,  460 

distribution 450 

groat,  of  Oiion 4'15 

gateous  nature  of. 447 

Xebular  hypothesis 403 

reached  by  reasoning  back- 
ward from  the  present.  . .  499 

conclusions  respecting 514 

Xcptinie,  history  of  its  discovery 353 

physical  aspect  of. 364 

satellite  of. .364 

Xewall.  his  great  telescope 138 

Xeirtiiti.  (11.  A.),  meteoric  showers 391 

Xeirtoii  (Sir  I.),  his  work 74 

laws  ot  motion 75 

theory  of  comets 402 

OU>ers,  hypothesis  of  the  explosion  of  a 

planet 324,  .320 

Orbits  of  the  planets 528, 530 


Parallax,  definition  of 165 

annual 170 

solar,  measures  of 171 

from  transit  of  Venus 175 

most  probable  value 200 

list  of  papers  <n\ 537 

stellar,  efforts  to  find 202 

list  of  measures 535 

Peirce,  rings  of  Saturn 350 

perturbations  of  Neptune 363 

theory  of  comets 403 

Phntoaphcre,  its  appearance 238 

light  and  prol)al)le  nature 263 

Pickerintj,  iuteusity  of  sunlight. 239 

37 


502 


INDEX. 


PAOK 

PlamtK,  the  spvcn,  of  the  nnoionts 14 

order  of  (lis tiiiice,  ancient 40 

modern 231,235 

laws  of  tlicir  motion fl'J,  03 

seciiinr  variations  of  orbits OS 

aspects  of. 235 

distances  and  masses 233 

of  other  suns 510 

supposed  intra-Mercnrial 100 

small,  till  <xt\\}  between  Mars   and 

Jupiter 823 

earlier  discoveries 324 

number  and  mass 3'-'S 

elements  of  orbits 530 

PlriadcK,  map  of 442 

JHuralitii  of  worlds 510 

Pole  of  the  heavens 10 

Precesxioii  of  the  equinoxes 19 

explained  by  Copernicus 62 

cause  of 88 

Proctor,  arranjjenient  of  the  stars 470 

Prominences.    See  Prohiheranccn. 

Protuhe ranees  of  the  sun 252 

spectroscopic  observation  of 254 

Ptoleini/,  his  system  of  the  world 32 

bis  answers  to  objectors 35 

his  relations  to  Coijernicus 58 

liis  catalogue  of  stars 413 

Pijthayoras,  crystalline  spheres  of 3 

his  supposed  system 4,  52 

Radiant  point  of  meteors 390 

liefraetion,  astronomical 300 

Reich,  density  of  earth 84 

RemMimj  medium,  indications  of 381 

researches  relating  to 382 

Rinps  of  Saturn 341 

Riitenlwuse  observes  transit  of  Venus. . .  294 

Roenier  searches  for  stellar  parallax 203 

Rouse,  his  great  telescope 131 

heat  of  the  moon 318 

Saros,  or  period  of  eclipses 30 

Sak'UitcN  of  Jupiter 330 

of  Saturn 351 

of  I'ranus 355 

of  Neptune 304 

Saturn,  the  planet 338 

asijcct  of. 339 

rotation  on  axis 340 

remarkable  spot  on 341 

rings 341,  350 

old  views  of 343 

phases  of. 344 

satellites  of 351 

Schiaparelli  theory  of  meteors 393 

Schot\fdd  catalogue  of  variable  stars. . .  ■  429 


Sehii'abe,  ppriodirity  of  snn-fipots 24S 

Seasons,  explanation  of 63 

Seechi,  temperature  of  the  sun 241 

on  the  snn's  constitution 205 

view  of  lunar  crater 315 

spectrum  of  nebula 417 

Secondary  si)ectriim  in  telescope 230 

Seidet,  photometric  researches 413 

SiriiLH,  brilliancy  of 413 

companion  of 439 

Solar  system,  rcla:ion  to  the  stars 101 

structure  of 231 

plan  of. 230 

Sjiectroscope  described 223 

Speetrutn  analysis  explained 227 

Sj)ltere,  celestial,  described 7 

circles  of. 140 

Spheres,  crystalline,  of  Pythagoras 3 

Stars  (see  also  Universe) 407 

arrangement  of,  in  space 400,  4TS 

binary  systems  of 43S 

catalogues  of 413 

changes  among  them 459 

clusters  of 441 

constellations,  formation  of 414 

descrijjtiou  of 417 

double 430 

light  of,  how  graded 411 

magnitudes  of,  ajiiiarent 410 

intrinsic 483 

number  of,  visible 410 

motions  of,  apparent 452,  484 

in  line  of  sight 450 

names  of,  how  given 415 

nearest 207 

new,  explanation  of 430 

nature  of 433 

observations  of  some 432 

parallaxes  of. 202,  535 

probable  orbits  of  some 470 

systems  of 470 

shooting.    See  Meteors. 

variable 420 

Stone  corrects  solar  parallax 199 

Struve  (O.),  changes  in  rings  of  Saturn. .  347 

inner  satellites  of  Uranus 350 

Struve   (W.)   investigates   stellar  paral- 
lax   '. 205 

parallax  of  «  Lyrte 207 

structure  of  the  universe 474 

Sun,  age  of 509 

appearance  of. 237 

atmosphere  of 240 

constitution  of. 258 

brightness  of,  as  a  star 483 

contraction  of,  probable 507 

distance  of,  moat  probable  value  of..  200 


INDEX. 


5o;j 


Sun,  distance  of,  methods  of  flndiiiEC I'JO 

gasi'ollH  theory  of 2fi4 

heat  of.  quantity  radiated 241 

how  maintained 247,  n05 

law  of  radiati(jn 5(11 

motion,  apparent  annual 14 

probable  real 4r)4 

parallax,  how  nieasnied ni-'2ni 

most  probable  value 'ini 

list  of  papers  on fills 

rotation  on  axis,  law  of 24'.» 

spots,  their  appearance 24-2 

their  periodicity 248 

appear  as  cavities 245 

8nrroiindiiij,'s  of. 251 

temperature 241 

Tdescripr,  origin  of ino 

Galilean  form  of Ids 

principle  of  construction  of 108 

ma^'nifyini,'  jjower  of 110,  lii'J 

aljcrration,  defect  of 110 

achromatic 114 

how  mounted  for  use lis 

rellectinj.',  how  made 121 

threat  ones  of  modern  times 125 

list  of  tlie  principal !)21 

Tlitiiiifiiin,  rif,'i(lily  of  the  eartli 2Us 

'y'i(/('.s.  how  produced 00 

friction  of,  retards  earth OS 

Thiw,  mean  and  apparent 1C2 

sidereal 150 

See  also  Calemku: 

Titm,  law  of  planetary  distances 2fi3 

Tntnuilf  (if  stars,  h,  w  observed 154 

of  Venus,  law  of  recurrence ITS 

old  observations ITS 

iul8T4 183,100 

in  1882,  where  visible 194 

of  Mercury 2S5 

2'ijcho  Brake,  hie  work CO 


rA<ih 

lTl\i(\h  Heirih,  catalofjne  of  stars 413 

i'liicifni;  structure  of 460 

stability  of,  not  necessary 490 

riaiiiin,  the  planet ana 

old  observations  of i)55 

satellite  of .'i55 

deviations  of  its  motion 358 

{'('iins,  ancient  theory  of  motion .'i9 

general  description 2Hl> 

phases 200 

supposed  axial  rotation 29t 

atmosphere 293 

spectrum 295 

visibility  of  (lurk  side 29(5 

satellite  of,  suspected 296 

\'o;)cl,  pliolographic  measures  of  sun's 

rays 233 

rotation  of  Venus 293 

spectrum  of  aurora 305 

views  of  Encke's  comet 307 

Vortices,  tlieory  of 72 

}y(tlkflr,  motions  of  Neptune 302 

Week,  days  of  the 46 

Wlit'utstime  revolviuf^  mirror 218 

Wimircb',  parallax  of  a  star 209 

ir«//,  i)eriodicity  of  sun-soots 248 

M'riijht,  spectrum  of  zodiacal  light 406 

Year,  sidereal  and  tro|)ical 20 

YiiUHfi,  constitution  of  the  sun 276 

researches  in  spectrum  analysis 257 

/.odiar,  definition  of 15 

sitrns  of. 10 

Zodiacal  liglit 2^9,  405 

Zi'iUiier,  law  of  sun's  rotation 250 

nature  of  photosphere 204 

intensity  of  nioonliglit 317 

theory  of  comets 402 


EXPLANATION  OF  TITE  STAR  MAPS. 

TiiKSE  maps  eliow  all  tlio  stars  to  tlio  fiftli  inafjjiiitiulo  inclnsivo  be- 
twoon  tbo  iioitli  polo  and  40°  south  docliiiatioii,  the  middle  of  each  map 
extciidiii<?  to  M'  dcclinntioii.  They  therefore  iuelude  all  the  stars  vhieh 
can  1)0  readily  seen  with  the  naked  eyo  in  our  latitu(h(s,  except  tho  very 
smallest.  They  are,  for  tho  most  part,  founded  ou  Ileis's  Atlun  Cwlestin 
and  tho  cat.iloguo  accompanying  it. 

To  reeognizo  tho  eonstellations  ou  tho  maps,  referonce  may  he  had  to 
the  descriptions  on  pp.  418-4'2G,  To  iind  what  eonstellations  are  on  tho 
meridian  at  any  hour  of  any  day  in  the  year,  it  will  he  necessary  to  cal- 
cnlato  the  sidereal  time  by  the  precepts  on  p.  151 :  tho  correspondin*;;  hour 
of  riglit  ascension  is  then  to  bo  sought  around  tho  niiugin  of  ^lap  I.,  and 
at  the  top  and  bottom  of  the  other  maps.  TIkmi,  if  Map  I.  be  held  with 
this  iionr  upwards,  it  will  sliow  the  exact  position  of  the  northern  constel- 
lations, Avliile  on  Maps  II.-V.  it  Avill  show  tho  position  <»f  the  meridian. 
Each  of  these  four  lust  maps  extends  about  from  tho  zenith  to  the  south 
horizon. 

The  several  dates  on  the  ecliptic  show  tho  positions  of  the  sun  during 
its  apparent  annual  course  as  descril)ed  in  part  i.,  chap,  i.,  ^J  3,  and  ex- 
plained on  pp.  54,  55.  The  apparent  path  of  tho  moon  in  1877  is  marked 
out,  in  order  to  illustrate  v^  (5,  p.  '21. 

To  illustraco  precession,  tho  position  of  the  equator  2000  years  ago  is 
shown  on  Map  II.,  where  it  can  be  coiupared  with  the  present  position, 
marked  0°  ou  tho  sides  of  Maps  II.-V.  For  the  same  object  the  circle 
which  the  celestial  pole  seems  to  describe  around  tho  polo  of  tho  ecliiitic 
iu  25,000  years  is  shown  on  Map  I. 

The  small  circles  marked  here  and  there  on  the  maps  show  the  i^ositions 
of  the  more  r(>markable  nebula)  aud  star  clusters,  a  list  of  which  is  given 
iu  No,  III.  of  the  Api)eudix. 


Map  I.— The  Northern  Constellations  within  50°  of  the  Pole. 


Map  II.  -Southern  Constellations  visible  in  Autumn  and  Winter, 


Map  III— Southern  Constellations  visible  in  Winter  and  Spring. 


9rn  Constellations  visible  in  Spring  and  Summer. 


Map  V.-Southern  Constellations  visible  in  Summer  and  Autumn. 


ADDENDUM  II.— THE  SATELLITES  OF  MARS. 

While  tLis  work  is  passing  tbrougli  the  press,  one  of  the  most  remark- 
able telescopic  discoveries  of  the  century  has  been  made  by  Professor 
Asapli  Hall,  of  the  Naval  Observatory,  Washington.  On  the  night  of 
August  11th,  1877,  be  was  searching  in  the  neighborhood  of  Mars  for  a  pos- 
sible satellite,  and  found  a  small  object  about  80  seconds  east  of  the  ])lanet. 
Cloudy  weather  prevented  further  observation  at  that  time,  but  on  the 
night  of  the  16th  it  was  again  seen,  and  two  hours'  observation  showed 
that  it  followed  the  planet  in  its  apparent  orbital  motion.  This  showed 
conclusively  that  it  was  not  a  fixed  star,  and  must  therefoi-e  be  a  satellite 
of  Mars,  unless,  by  chance,  one  of  the  group  of  small  planets  between  Mars 
and  Jupiter  happened  to  occupy  this  position.  Examining  an  ephemeris,  it 
was  found  that  the  small  planet  Europa  was  calculated  to  be  only  2  or  ^ 
degrees  distant  from  Mars ;  and  if  the  ephemeris  were  erroneous  by  this 
amoimt,  the  object  observed  might  be  this  very  body.  This  seemed  ex- 
tremely improbable,  but  the  possibility  of  it  was  sufficient  to  deter  Pro- 
fessor Hall  from  announcing  his  discovery  until  the  question  could  be  set- 
tled by  another  observation.  A  rough  calculation  from  the  observed  posi- 
tions of  the  satellite,  and  the  known  mass  of  Mars,  showed  that  tlio  period  of 
revolution  would  probably  not  be  fiir  from  29  hours;  and  that  if  the  object 
were  a  satellite,  it  would  bo  hidden  during  most  of  the  following  night, 
but  would  reappear  near  its  original  jiosition  towards  morning.  On  the 
other  hand,  if  the  object  were  the  small  planet  Europa,  it  would,  on  the 
next  evening,  be  a  little  south-east  of  the  planet. 

The  following  night  was  beautifully  clear,  and  when  Mars  rose  high 
enough  to  bo  well  seen,  the  telescope  was  pointed  at  it.  A  snuill  star  was 
soon  seen  quite  near  the  computed  position  of  the  hypothetical  small 
planet,  while  no  satellite  was  visible.  But  a  few  minutes  of  observation 
with  the  micrometer  showed  that  Mars  was  passing  by  this  object,  and 
that  the  latter  was  therefore  a  fixed  star,  and  not  th<5  moving  object  seen 
on  the  preceding  night.  The  appearance  of  the  satellite  was  therefore 
looked  for  with  much  confidence,  and  at  four  o'clock  on  the  following 
morning  it  emerged  from  the  rays  of  the  planet  as  predicted,  so  that  no 
reasonable  doubt  of  its  character  could  remain. 

IJut  this  was  not  all.  The  reappearance  of  the  satellite  was  followed  by 
the  appearance  of  another  object,  much  closer  to  the  planet,  which  proved 
to  bo  a  second  and  inner  satellite.  The  reality  of  both  objects  was  abun- 
dantly confirmed  by  obsevvations  on  the  following  nights,  not  only  at 
Washington,  but  at  the  Cambridge  Observatory,  by  Professor  Pickering 
and  his  assistants,  and  at  Cambridgcport,  by  Messrs.  Alvau  Clark  &.  Sons. 


566  ADDENDUM  IL—THE  SATELLITES  OF  MARS. 

Tlio  most  extraordinary  feature  of  the  t\vo  satellites  is  the  proximity  of 
the  iuncr  one  to  the  planet,  and  the  rapidity  of  its  revolution.  The  shortest 
period  hitherto  known  is  that  of  the  inner  satellite  of  Saturn — 22  hours 
'61  minutes.  But  the  inner  satellite  of  Mars  goes  round  in  7  hours  38  miu- 
ntos.  Its  distance  from  the  centre  of  the  planet  is  about  6000  miles,  and 
from  the  surface  less  than  4000.  If  there  are  any  astronomers  on  Mars 
with  telescopes  and  eyes  like  ours,  they  can  readily  find  out  whether  this 
satellite  is  inhabited,  the  distance  being  less  than  one-sixtieth  that  of  the 
moon  from  us. 

That  kind  of  near  approach  to  simple  relationships  between  the  times 
of  revolution  is  found  hero  which  we  see  in  the  satellites  of  Jupiter  and 
Saturn.  The  inner  satellite  of  Mars  revolves  in  very  nearly  one-fourth  the 
period  of  the  outer  one,  these  times  being. 

Outer  satellite 30h.  14ni. 

Oiic-foiii'th  this  period 7h.  33Jin. 

ruriod  of  inner  satellite 7ti.  3Sm. 

Tliese  satellites  may  also  bo  put  down  as  by  far  the  smallest  heavenly 
bodies  yet  known.  It  is  hardly  jiossible  to  make  anything  like  a  numeri- 
cal (jstiinato  of  their  diameters,  because  they  are  seen  in  the  telescope  only 
as  faint  points  of  light;  and,  having  no  sensible  surface,  no  such  thing  as 
a  measure  of  the  diameters  is  possible.  The  only  datum  on  which  an  esti- 
mate can  be  founded  is  the  amount  of  light  which  they  give.  The  writer 
judged  the  magnitude  of  the  outer  one  to  be  between  the  eleventh  and 
twelfth.  According  to  the  estimate  of  ZoUner,  Mars  itself,  at  this  opposi- 
tion, is  three  magnitudes  brighter  than  a  lirst-magnitude  star.  The  differ- 
ence of  brilliancy  between  Mars  and  the  enter  satellite  is,  therefore,  repre- 
sented by  thirteen  or  fourteen  orders  of  magnitude.  From  this,  it  would 
follow  that  Mars  gives  from  200,000  to  r)00,000  times  as  much  light  as  the 
satellite ;  and  if  both  are  of  the  same  light-reflecting  power,  the  diameter 
of  the  satellite  would  bo  from  6  to  10  miles.  It  may  be  as  small  as  5  miles, 
or  as  great  as  20,  but  is  not  likely  to  lie  far  without  those  limits.  The 
inner  satellite  is  much  brighter  than  the  outer  one,  and  its  diaaieter  pi"ob- 
ably  lies  between  10  and  40  miles. 

The  following  are  the  first  rough  elements  of  the  apparent  orbit  of  the 
outer  satellite,  supposing  it  circular.     The  distance  of  the  iuner  one  is  33". 

THE  OUTER  SATELLITE. 

Major  semi-axis  of  apparent  orbit  seen  at  distance  [9.5030] 82".5  ±  0".5 

Minor  senii-axis  of  apparent  orbit  seen  at  distance  [9.5!).30] 27".7  ±  2". 

Major  semi-axis  of  orbit  seen  at  distance  unity .S2".3 

Position  angles  of  apsides  of  apparent  orbit 70°,  250°±2'' 

Passage  through  the  west  apsis  (;;-=250°),  Aug.  19,  IC'.C,  W,  m.  t. 

Period  of  revolution 30''  14"  ±  2™ 

Hourly  motion  in  areocentric  longitude 11°. 907  , 

Inclination  of  true  orbit  to  the  ecliptic 2r)°.4  ±  2° 

Longitude  of  ascending  node 82°.8±3° 

Pobitiou  of  pole  of  orbit  iu  celestial  sphere Long.  352°  .8       U.  A.    316.1 

Lat.     -f  64.0       Decl.  -|-63.8 


